Experiences I've had with this; succeeded the hack on three enemies, only one got made a Resistance MEC.
What I learned; when it says MEC, it means MEC. The white ones. Not the red Advanced MECs, not the green Archer MECs, not the blue Longbow MECs. JUST the standard white ones.
Which is weird, because Resistance MECs have higher hit points that standard MECs, being closer to Advanced MECs in many regards. I'm hoping there'll be an update at some point that allows other MECs to count, even if they all become the same thing afterwards.
As an aside, white MECs are pretty easy to succeed that hack on, at least.

If this is true, that is a shame, as it makes the Full Override a lot worse. Basic MECs are only threatening early on, later they get shredded by things like Mutons and advanced ADVENT forces. Taking Advanced MECs would help, since those stand up better, but even they go down.

I would really love to be able to take Sectopods to resistance HAvens, since, well that would be incredibly awesome. Maybe not super balanced, but hey, who wouldn't want to pilot a Sectopod and annihilate enemies?

Well, you can still Full Overide them and have them for a whole mission. It'd be similar to Dominating Gatekeepers, which is a favoutire of mine; ADVENT have no realistic way to deal with all that armour!
Hopefully it will allow different MECs to count in a future update, or someone will work out a .ini edit.