If you're running single-player, just 'Edit Mod Pack' in the launcher *before* you fire up Unleashed. Move PowerConverters from the right side (disabled) to the left.
If server, just remove the .disabled extension in the mods folder. You can do the same on your client, too, but the GUI method is easier.
As for complaints, here are the things people have problem with with Power Convertors, from roughly worst to least offending:
A. Steam (especially Tesseracted / Ender Tank'ed steam !)
B. Tesseracted MJ when net effect is Tesseracted IC2
C. IC2 <--> MJ. Some modders want you to use their power gen ecosystem to drive their machines and don't like the competing/other power gen systems
D. Just a boring block, like solar
Again, that's roughly worst to least offensive, as far as I can tell.
Personally, I use PowerConverters end-game when I'm consciously trying to avoid engine/block spam. I will stop building 36HP boilers when I have around 10 or so. 180 Industrial Steam Engines is a lot of engines. I've proven I can do it. Beyond that, I'll be server-friendly, so HV Solar or Ultimate Solar into Power Converter to produce still more MJ is more server-friendly. Boring, yes, but I'll go visit my 180 Industrial Steam Engines if I need to be reminded of the sight and sounds of MJ power production.