BuildCraft version: 7.99.21
Forge version: 14.23.5.2807
Link to crash report or log: https://pastebin.com/7tzcUWtB
Singleplayer or multiplayer: neither- fail to load
Steps to reproduce: attempt load
Additional information: Removing BC 7.99.21 allows the pack to load
Re-adding BC 7.99.21 results in failure to load.

It was an issue with the advanced tooltips being enabled in options.txt
Set to false and good to go!

If it crashes then it's a bug. It doesn't matter what your configuration is.
However this is a duplicate of #4261, so I'll track it's progress there instead.