Well, from what I've read, Optifine cuts into the hardcoding of MC - which in turn, can cause other issues at random. For instance, when using Optifine EVEN with the patch that everyone is raving about (which was created for shaders), you still have things that render invisibily, namely entities, such as chests, mobs, and even self portrait. Everything else seems to work fine with the newer FTB MC 1.6.4 modpacks. So, if you are willing to deal with invisible things (which, I couldn't stand it after a while), than, you can technically use Optifine....I have removed the 'fix' link from my texture packs as it isn't something I would use.
That's amazing Aedaeum...I was also informed that sp614x will not change his code to conform with mod support/MC support etc. Therefore, it's half-supported with MC and it's mods, I guess, in a way
idk, maybe MC 1.7+mods could be different, but, doesn't look good. Guess we shall see how it plays out