Special crew has a lot secret mechanics on how they function, because their tooltips barely shows anything about ther real bonuses.

As first thing to make clear, they get 'experience' by killing an enemy, dealing damage to it doesn't matter, they also get experience if they controll a tower.

- by themselves:
DPS: 3 -> 3.6
RANGE: 8 -> 9.6
SPEED: 2.5 -> 3
HEALTH: 30 -> 36
(as you can see, they're 20% stronger in all aspect compared to regular crew)

- in towers:
they give a simple 20% damage boost for towers
(it is recommended to swap them around to lower kill potential towers, so they get the damage boost too, while the higher kill potential tower lvl up other crew, so you can stack up special crew too as a backline defense)

- in extractors:
+1 building material/ sec
(so with the perk, you can get +9 resource/sec from a single extractor)
* it's not written in the tooltip of the extractor, you'll see +7 (+8 with perk) / sec, but in reality, it will give you an extra building mat every second
* as a new experience with RAMBO... special crew automatically comes out from extractor if it's attacked, which isn't too good for their health, so either don't use them in frontline extractors or use PROTECTION perk combined with them

- reserch lab, uplink and arms factory
as far as I've tested, they don't get any bonuses from having special crew handling them

GOOD AS NEW
-50% repair cost, nothing extra

REFOUND
-you don't lose money by repositioning towers

QUICK SUPPORT
-50% faster crew requesting (it actualy is better in rushing crew, than +1 crew/order)

HURRY UP
-faster reserch, you should swap it if you're done with you most important reerches to something else, it's only important on certain maps, where you don't start with missile and you need it fast, but for that, you might want UNLOCK, that gives you all building baseline

PROTECTION
-only usefull if extractors are actually in the way of zombies, otherwise don't pick it
(sometimes you will ony know if it's worth it or not if you are inside the map)
* my personal favourite in challenge mode, since there are often extractors in the front and it helps ignoring their defense, makes it easy to turtle up at the lander

REINFORCEMENT
-it's a value perk, best way to use it is to order +1 crew each time, you spare at most 120 building materials/ 60 sec, so you're better to pick TURBO, if there are 3 places for extractors, because they give you +180/ 60 sec and so on, if you have even more

COFFE TIME
-incredibbly strong perk, it actually gives you effectively +210 building mats/ extractor (lvl 3 and 240 if manned up by special crew)/ zombie weave; the extra time also gives you sure preparation time to get all extractors in most map to lvl 3 without risking being rushed down at start

SUPERWEAPON
-50% cost reduction on superweapons, only usefull if you use all of them realy often (like 3-4 at each weave), otherwise it's better to pick resource boosting perks

BOMBASTIC
-same as superweapons

TURBO
-one of the best resource boosting perks, only coffe time can be a bit better if you need more reaction time, also makes it better to spam out all resource extractors

ENERGY BOOST
-only good if you minmax and want to get 5 slot energy hubs fast, then it's better to switch to this perk, or in realy long maps, at the end, to get even more tower slots

NO WORRIES
-400 resource caches are dropped every 60 seconds randomly on the map, a crew must go out to pick it up, it despawns if no crew picks it up after 60 seconds, it despawns; spawn countdown only starts after the current one is picked up or has despawned
so it's theoretically gives at most 6+ 2/3 resource/ sec, but it's better to calculate with 6 at most
* has a chance to spawn on top of a building... it can't be picked up from there, which is a serious desadvantage, but it's still quite a good resource boost

CATAPULT
-if a building is destroyed, crew escapes; the exact mechanic is that it escapes through a capsule and it lands next to the lander after ~10 seconds and goes into the lander automatically

PPOWER UP
-simple reduces towers upgrade costs

BOUNTY
-personally haven't tested, but since there are realy lot small zombies, that baseline only give 1 building mat, the +2 effect on maps, where small zombies swarm, this perk can be realy good, but only if you swap it constantly back to other resource boosting perks, because there are only ~1000 zombies at most on 1-19 maps, so other perks are just better mathematically, the 20th map however is a different scenario, there, you can get serious ammount of resources from this perk, but if you realy want resources on that map, you can just use NO WORRIES, because until you don't use up all the monoliths, you won't descend to the next lvl and you can farm as much resource caches as you like... well, that's still quite a boring thing, but technically better :)

RAMBO
-you still start with normal crew, that needs to be leveled up, but every new crew you order will start as special crew, so it's quite usefull
* important fact is, if you already have incoming crew and you switch into this perk, they will be special crew still, even if you switch into it just a second before they arrive

UNLOCKED
-unlocks all techs, but also blocks their reserch options, since they're counted as finished, but after switching the perk, you can reserch them still
* an important fact is, that the perk is only required to build the towers, to upgrade them, it's not required, so it's actually possible to build up your advanced tower defences with lvl 1 towers, spaw out the perk and later upgrade them

CHALLENGE
-all weaves are instantly summoned and you get the max bonus for calling them early

DUCK & COVER
-meteors stike down on the map randomly (even completely outside of the combat zone
haven't waited them to strike on a tower maybe or sure-hit a mutant, but they probably deal a lot of damage... not a too usefull perk

HELL YEAH
-you get a button at the top of the upgrade option for the gun turret, with that, you can man up the tower by yourself, no actual crew required
you won't get any map messages, so it can easily cost you the game if other lanes are not properly defended, needs a strong computer or low resolution setup
the tower gets 'infinite' range and you can kill most zombies by one hit, with right click, you can even zoom in for better aim; personally not tested, since all lesser zombies are oneshotted anyways, but headshots might deal extra damage to stronger zombies if they don't die from a single hit
-there's a single lane map, you can practically defend yourself on that map with this perk with a single tower if you're good enough

Perk swapping
perks, that has a certain cooldown time are instantly activated, when they are equipped, also you should wait for their 'last' activation, before swapping them out

Cliffs
Towers can attack at every elevation within their range, althou there are only a few maps, where you can use this, this trick can be quite effective on that few maps with some long range towers

Early game perks:
If you're on a long map with lots of weaves, then you can go for a min-maxing build even, with tech rush perk, then swap to other pekrs, like Rambo or resource boosting perks.
You can even consider Rambo an early game perk, if you rush out 12-15 extra crew with Rambo, then switch to something else, so you have all the crew as special force (don't forget to swap out extractor crew early, so your starting crew can lvl up fast too from towers).
Coffe time can give you serious extra preparation time, but in the later parts, you'll be simply too strong to consider that perk usefull at all.

Perk combinations:
- Cheaper upgrades, then swap to 100% refound from selling -> you're practically making free money. Althou you should have a lot of buildings to upgrade, then sell all of them… I don't think this combination is viable, since if you've already built the towers, you already had enough money for them, why sell them?
- All building unlock - cheaper upgrades, you slam down all lvl 1 towers in position, maybe even use a +1 energy upgrade too, then swap to cheaper upgrade and upgrade all of them to max.
You spare all the reserch time and money, you have all towers in place and cheaply upgraded too.
You can even swap them around to upgrade energy hubs to max, then unlock again (700 is realy expensive, but 525 is a much more bearable cost for lvl 3) to lay down the new turrets, then maybe if you have enough resources, you can reserch the techs you need (best to leave 6th slots only a single tower type, swap to +1 energy slot to max out defenses.