I'm pretty sure that I'm probably the millionth person to point to this flaw, going all the way back to when Adobe first added envelope tools to Illustrator. Curiously, I never bothered to complain about it over all these years until now. What sparked my complaint now? It was probably just me laughing at myself for how I've gotten around this problem. I keep a copy of Coreldraw demo. I make envelopes in Coreldraw which are are accurate. I screen capture the result, run it through a high quality vector tracing software (Illustrator's vector tracing is good sometimes but not when going for perfection), then import that result back into Illustrator for further work. I find it kind of funny but also kind of ridiculous. Why hasn't Adobe fixed this flaw after all these years? I wonder if its really just asking too much for Adobe to finally tweak this thing so that it makes envelopes that aren't warped? Photoshop by the way has the exact same flaw.
I definitely have tried that as well. It has the same flaws. It requires a lot of fiddling and if you want to make an adjustment, it requires a lot more fiddling. If I were still using a Mac, I would go back to this old program that we used to use called Typestyler. The company I used to work for used it for the singular purpose of doing these types of envelopes. Its still around, but still Mac only. Oh well, back to work.