All protocols are invented at some point. Telegram did a terrible job marketing this one but it has been a long time now and the only issue I ever heard of was fixed some years ago. It's still not exactly pretty, but then look at TLS and I'm actually quite okay with mtproto.
The real issue is that mtproto is never used. It isn't implemented in most clients for no apparent reason ("can't keep state for encryption keys!" is the usual excuse - dude you keep my login token what's the big deal here) and if you try to use it, it doesn't sync between devices. One of the core selling points is a solid desktop experience.