Well, lets share some tips and tricks about tech building system in the game.
It became more complex than in standard TerraTech. I do not pretend to be an expert, so I would appreciate corrections and hints.
in () is my assumptions


There are 2 main limits:
1. Reactor Capacity
(limit of added blocks)
2. Cab Efficiency
(limit of added wheels)
UPD (Not 100% sure that's accurate. I think it modifies absorbed cost when using modifiers like weapon mounts, etc. I noticed that a weapon on a mount seems to cost less than the -3 reactor capacity the weapon mount implies.)

One of key feature are modifiers:
1. TURBO
Weapon - Increases fire rate
Wheel - Increases acceleration and maximum speed
(Does what it said)
2. AMPLIFY - Increases output
(It makes weapon more powerful, deals more damage. Assuming will work on lasers, booster etc.)
3. NANOBOT REPAIR - Increases repair speed
(faster block HP repair, if they are not destroyed. Then only R will help)
4. RECYCLE - Blocks have a chance to not consume reagents
(Weapons can fire without consuming a bullets. Very useful. Assuming booster etc will also use less, lasers not affected)
5. SUPERCONDUCTOR - Reduces power consumption
(lasers and similar parts uses less tech power)
6. ARMOUR - Decrease damage taken

There is tech temperature indicator which can be useful. You can add parts above reactor capacity limit, that will decrease some of your techs characteristic depending of overheat. But it can be balanced by arctic biome coldness for example. So tune your tech for different biomes.

There are different conditions for mods. Modifier can apply to all blocks of the tech, directly connected blocks or some specific. So, the main goal in creating an effective tech is to take into account the features of modifiers and arrange blocks in a special way.

Press F to mirror tech part.

Batteries can add tech power and good for lasers (note that lasers have less attachment points), cargo can add more storage and good for ammo weapons.

Some modifiers reduces reactor capacity of weapon or wheels like weapon mounts, shafts and frames. Late game cabs have specific conditions for modifiers. So use all those features to your advantage.

I recommend to use armour with most HP. 500 is great for 5 reactor load. Not sure why other armour has much less HP.

Wheels are something different. Assuming large wheels can't swim through sea. Small and middle wheels couldn't handle lot of weight.

Shield projector and weapon mount can be tricky. Do not block the aiming area and test your tech before going out.

Here is an example of modified Witch Fire. This weapon has many attachment points, so it's easy to make a lot of mods. Currently I got 2 weapons with: 3 turbo 3 amplify and 8 recycle. It became very powerful and uses less ammo (about 2x less).

An example of wheel with turbo modifier