I dont know if that would actually damage SOBDE as people would basically hate you for being OP, well more than usually XD
Battalions bonus are so battalions arent literally, what's your favourite colour, mines blue so I'm 501st .
Also a lot of non SOBDE battalions have requirements before you can access their better classes. Mainly GM and Rancor comes to mind. GM need GMACT, they may not be as hard as SOBDE, I've never been to any, but I know of many previous GM who, along with ARC successfully become RC.
Saying SOBDE earn them is kind stupid as you dont join RC as an enlisted like your join a batt and earn trainings and experience and shit.
SOBDE takes already capable soldiers and gives them good shit. The tryouts isn't a trainings it's an elaborate test of varing difficulty based on whether you are good at what ever elements are tested.
SODBE is hard for the facts of its exclusivity, status and reputation, in lore anyway.
GM is hard as they where the most versatile troopers that could do anything, never surrendered, no matter the odds. [New Bornalex, battalion if B2s, weapons disabled for the GM, used power enhancing prototype suits to first fight the droid, 99% casualty rate =victory (slight exaggeration)] GM was feared.
Rancor and ARC is hard as it was a training that changed the perception of the soliders to be more independent and smarter as well as dealier. That's why most commander where ARC trained and many of them are some of the best fighters you see in the TV show and comics and other media.