Probably only packs that used 'approved mods' or whatever the term is. I went on a bit of a downloading spree the first time I used the new launcher
For the curious, FtB says they only use mods that the developers give them permission to use and distribute, and they only use mods from reliable download sites to prevent malware, thus the use of the Curse repository.
Sometimes mod devs will withhold a mod for personal reasons (not ready for primetime, too buggy, no time for upkeep, etc.) or, in at least one incident they didn't want their masterpiece used with certain other mods. This last was going on when I first came to Minecraft and went so far as one dev intentionally causing their pack to fail spectacularly if another dev's pack was installed.
They also prefer not using a lot of mods that modify the actual core code of Minecraft, those will tend to break when newer Minecraft versions are released and having to patch Forge to fix things was becoming a nightmare (as I understand). In fact I've read that if 1.12 is indeed a stopping point version, the Forge guys may try to do a complete rewrite of Forge to get rid of no-longer needed code and optimize the rest.