Last but not least: if you want "hard" survival why don't people go explore the twilight forest? The boss mobs there are tougher than anything else in Minecraft really.
I feel this is likely aimed at me. Yes it was the first thing I did and I loved it, my main issues are (This post will cover my opinions on twilight forest: TL;DR, the mobs are too weak aside from the hydra)
Liches are too easy unarmed - Most of their hp is gone just form deflecting very easy to deflect bolts. I believe these guys should at the very least have more health, and preferably would also move around in p1/p2, not just teleport. This would make this more like a boss and less like a free ranged weapon + ender pearls. The lich should be the hardest boss, since the staves are by far the most powerful item these bosses can drop.
The naga is also flawed. If you have iron armor and a full health bar you will simply regen your health before the naga comes back for round 2. It's like fighting the ender dragon in full diamond enchanted on an infinite length platform. You just wait for it to attack you and hit it, then your health auto regens from it's hits. It also has very little health once again.
The hydra is the only real interesting boss here. I would prefer if it started with 7 heads, since this would destroy the localized terrain much quicker (and hydras are very hard to beat when there's no stable terrain around them, as I found out after a friend on my server died endlessly causing a field of explosion holes falling into caves). I feel the hydra is fine in general, it is balanced for what it drops (hydra chops)
The general mobs in the twilight are not dangerous - you have mosquito swarms that do 1-2 damage base it appears, and die in one iron sword hit. You have kobolds that are 1-2 shottable zombie pigmen that are inherently hostile. You have redcaps that... i don't know what they're meant to do.
Honestly, the wither is comparable to the hydra in lethality. The wither skull grind is also in my opinion worse than anything any mod on FTB has implemented.