WotC itself shows that repeatedly they themselves have trouble following there own rules and making fun balanced content (in this case I don't even just mean balanced like a new class... but an adventure that is fun in all three listed types of play too) and as such I have had to think through each new book (it is easier with slower 5e releases) about if I will let them into my games or not... but mostly I do allow WotC stuff.
See the problem here is you're thinking just of sourcebooks.
It's absolutely true that when it comes to sourcebooks, i.e. player options (races, classes, subclasses), WotC do a better job of providing balanced and playable material (IN 5th EDITION!!! Not in 3rd, say...) than 3PPs. But that said, it's not at all hard to find races, classes and subclasses that actually mesh well with 5E and are balanced (you will encounter plenty where people don't understand how to write rules or what the action economy is, though). The average skill of people doing 3PP game design for 5E is a hell of a lot higher than it was for 3E (that also goes for the official designers of course).
With adventures it's a bit different. The better-quality 3PP adventures are, imho, much better-designed, and much more "runnable" than most of WotC's adventures (which seem to be designed to read and then spend huge effort filling in a ton of unnecessary blanks, which don't even really let you customize for your party, just do a lot of work). They also have a much greater diversity of adventure design than WotC offers.
Monster-wise I'd say some 3PPs put out better-designed and balanced monsters than WotC, at least up to MotM.
Also with setting design, many 3PPs do as well as WotC, and most assuredly provide setting types WotC is no longer willing to support (i.e. settings which aren't kitchen sinks and so on - yes there was Theros which vaguely suggested maybe if you wanted to, limiting things, but AFAIK, that's it)