FTB launcher - is a Minecrfat Launcher. Ans JUST a Minecraft launcher. Curse Launcher is a conglomarate of a multitude of Features a Minecraft-launcher does not need. also you cant install it where you want it to be installed. that was a big selling point of the original and curse destroyed it. I may repeat myself, but the curse/twitch launcher is not an option if you wnat to play minecraft whitout that WoW launcher that curse is and will always be. i dont need another steam like application. we already have origin, u play, nVidea experiance (yes they do have a 'game launcher'), AMD Raptr, to just name a few.
I dont need another add me friend thingy with chat and voice - i have discord for this. also teamspeak. the latter more stable and more usefull.
oh, and i HAVE to have an curse account linked to the launcher, otherwise i cant even use it. FTB just works.
so, to have a 'newer' client i have to downgrade in user friendlieness, have to downgrade to a programm that you can't install where you want it. (remember folks. the C-Drive is NOT the Drive to install ALL the things, go read it up) and i have to deal with a buttload of crap that adds features that are plain simply not part of the minecraft launcher, but of an Suit that is uncalled for.
tl;dr: The Upgrade to Curse is an downgrade overall.
I'm not a fan of using Curse as a launcher, personally, but the mod repository is unmatched in usefulness when building a custom pack. I agree that Curse is a lot of bloat that I don't want or need in most cases - the amount of precious system resources it requires to do these things I'm not interested in is a major turnoff. I've moved to MultiMC as my primary launcher, since it is light while still having dev support. My remark was simply a warning that any issues you run into using the now defunct Legacy Launcher will likely never be dealt with.