Whenever the user is involved, whether Tiny or some other editor, in my experience the user will @#$$%$ it up. It’s an axiom. Yes, Tiny has its issues, but clients are even worse, and if they’re slightly techie, in can be yet worse that that, because they want maximum control, such as the awful colors from their brochure.

In several cases I’ve seen, engineers are the funniest of all. The web is not Microsoft Word. So part of it is education. I’ve written an article or two about this myself, because it’s a universal problem with CMS systems.
Much of this stems from the fact that your editor will NOT look exactly like what they get when they save, due to aforementioned CSS, etc. And trying to make the editor match their site is, IMO, a fool’s errand.
If they want to make a table, pfffff, forget about it, they’ll be phoning you.
So I try to make a reasonable compromise. I brew up CSS for paragraphs, various headers, bold, italic, and a few other basic things so they get something palatable without resorting to colors or other micromanagement. They just type and go, and stick in a very few extras.
I argue that if they need more graphic elements and more control, they should have the same graphic design training you have, or they need to have a geek do it. And I have arguments from clients about this, but hey, too bad, that’s reality.
All that dire stuff aside, there are other things you can do, too, like having TV’s with instructions for complex things that go into a template very precisely, or to make plain text work OK. Then it’s styled on the back end by me. Or you can try the FCK editor. Some like that better. But no easy answers to this complex a problem. And other CMS systems may have an editor you’ll like better. But Tiny is everywhere, probably due to its portability.