Weeeeeeellllll.... I like GregTech, to an extent. What do I mean by this? Well, I grew accustomed to GT in MC version 1.4, when it was still at a point where it wasn't easy to reach end game, but it wasn't stupid either. I didn't replay gregtech until a few months ago. And since I started I have had to minetweaker a LOT of recipes to make my mod pack playable with GT.
Lets start with vanilla... WHO THE HELL THOUGHT CHANGING VANILLA IS A GOOD IDEA! I will end my argument there for that part of Gregtech
The Bronze age, yeah, lets skip that. A waste of time if you ask me.
Then I don't really mind too much. I still use Mekanism Circuits (which I oreDict.'d to work as IC2 circuits) because GTs recipe for IC2 circuits is a little stupid. But other than that I enjoy all the machinery added, and what you can do with it. But it's just a PITA to start with nowadays, before you could jump straight into it once you had been mining for a little while.
Hunger Overhaul. I love this mod, only with Pam's HarvestCraft installed, as it makes food more important again. Remember when hunger was added in beta 1.8, you were constantly checking you had enough food. Well now you have to do that again, because hunger is too easy in vanilla
RotaryCraft. SKIP! Next! I don't like this mod, just because it is ridiculous. Stuff keeps exploding, and it is pretty annoying
Lycanite Mobs, I just hate because they screw you over early game. If you join a server with it, and it is night time, expect to die.. A LOT
Infernal Mobs, I am on the fence about. I like it in the nether, as it makes the nether hard. But it makes it a little too hard. If you are playing with just vanilla... all I can say is good luck
Thaumic Tinkerer. I love this. It is worth the grind. You need a lot of time and patience, but being invincible is worth it.
I can't think of other "grindy" or "hard" mods, but those are the main ones I have opinions on