I don’t have any issues with it. I’ve managed countless campaigns with multi-million dollar ad spend through it and managed thousands of client accounts through it for single companies.
Those that do big business with Facebook (and others) generally build their own systems to manage campaigns through the API. If you’re upset about how BM is setup, make your own.
I’m curious, what large-scale paid advertising platform are your familiar with and like/love, if you hate Facebook Business Manager so much?