Reported by CritFlaw in the buildcraft discord, with this log: https://gist.github.com/AlexIIL/de12ef9171a44d12e0764d272243b715

Essentially it just stops loading after it gets to "Completed: Ender Tweaker" (95%) and then just... stops.

I noticed this behaviour with Optifine. Also noticed that the latest.log just ended and didn't show any reasons immediately before the stop. After removing Optifine, it worked again. Although later I saw that after waiting even longer, it managed to load with Optifine installed as well.

Isnt an issue anymore. Just does take awhile to load

I did some investigation a while ago and never wrote down the results - sorry.

It turns out this is caused by mods checking to see if the "shift" key is down while creating their tooltips (likely for JEI or something else), which gets blocked by the CLS thread for most of the time (as it's rendering). One solution to this would be to hack into the method to check to see if the shift key is down, and just assume not if CLS is currently rendering.

I noticed the same bug making a mod pack... I at first thought it was due to the fact that EnderIO has a warning about some missing Class but then I realized it always did that. So I tried the Optifine fix that @Liiw suggested and it worked.