The Senate Guard any better?

Most of them were just Padawans or young Knights who were recruited either because of them demonstrating a disillusionment with the Jedi or were coerced thru torture, not because of how formidable they were. They often face opponents of equal or greater skill, and they were considered expendable by the Vader and Emperor; useful tools for rooting out a stray Jedi nuisance here and there, but leaving the big dogs for Vader to handle.

Reminds me of the purpose that Ashigarusha being the expendable soldiers to deplete all of the enemies ammunition and weaponry until the real soldiers arrived.

Palpatine does not want them to become a threat.

And they are still pretty dangerous as long as they don't face enemies like Ahsoka.

Not all of the Inquisitors were weak. The Grand Inquisitor wasn’t weak by any means.