I know FTB Ultimate is a very old pack, but I recently started playing it again and may have just lost my world.
I placed a Wooden Redstone Pipe next to a Thermal Generator and it crashed my game. It won't reopen my world without a crash screen every time I try to load it up. I tried removing Buildcraft from the mods folder temporarily but it would not let me open the game from the launcher. There are also no backups as far as I can find. My most recent attempt let me log in, but all I could see was my HUD before it crashed again.
Full Crash Report:
This is my 4th FTB Ultimate world that has been corrupted somehow, I'm beginning to realize why this pack was abandoned.