The powers Subaru receives have limited functionality.
First he only had the dark mist spell as his only skill, and he overused it until he lost the "inner gate" needed to do magic. The mist saved his ass a few times, but it's broken and gone now.
By killing Betelgeuse, Subaru gained that Sloth's power. Unfortunately he can only manifest like one invisible hand and land a single hit at a time. He also can't release a wave of sloth like the Bishop could.
He got drenched in dark dragon blood which burned and poisoned Cruche. The only useful thing he got out of it is transferring that blood from her to himself in order to save her.
He then gained Bishop of Greed's power, which is to become immune to any harm while he stops his heart. Obviously Subaru can't make any use of this ability yet.
His only consistently useful ability is him becoming master to Betty, but due to his limited power she can't really use hers to the fullest either.