I'm not talking about 3rd party splats. I'm talking about official WotC published products.
When you talk about Wizards stuff being used in Pathfinder, you are definitionally talking about third-party material. Paizo is the first party, you are the second party, WotC is the third party.
Let me get this straight. So, they rewrite the PHB (open content), so it's obviously not backward compatible with with the 3.5 PHB. It's been replaced. All of the other books are non-OGL, which they can't touch. What's left to be backward compatible with? Seriously?
They rewrote the OGL stuff, so you aren't going to be using any of the "core" 3.5 books. So they're claiming you can use those other non-OGL books and saying that works. What definition are you using?
Just because something mechanically "works" with 3.5 (or mechanically "works" with Pathfinder) doesn't mean it's not broken. Five minutes browsing just about any homebrew forum makes that clear. The backwards compatibility of Pathfinder is that it is extremely simple to port material forward from 3.5 to Pathfinder. That's all. Nothing more, nothing less. If it runs in 3.5, it'll run in Pathfinder with a minimum of changes, but Pathfinder won't magically make it less broken.
The "promise" you claim they have broken: "The Pathfinder Roleplaying Game has been designed with compatibility with previous editions in mind, so you'll be able to use your existing library of 3.5 products with minimal effort."
Note an absence of "also, the incredibly broken shit that Wizards regularly put into its books due to lack of playtesting, lack of knowledge about how the game works, or other factors will magically fix itself". You can use your existing library with minimal effort, and it will be about as good or bad as it was before. It's just that pretty much every Wizards sourcebook included at least one thing that was either incredibly broken on its own or had broken interactions with material in another sourcebook, and people justified it on the basis of "well, core is already broken beyond all belief anyway, so it doesn't matter if sourcebooks layer on more broken nonsense".
And when you actually get to the spells themselves some of them are nerfed, but mostly in meaningless ways. The ones that actually were nerfed were quite meaningless on a power scale of things. None of this matters though, as you still have at least 1 good spell at each level meaning you're no less effective, just less interesting.
This right here is the crux of the problem. The attempt was made to nerf wizards in the spell lists, but the guys doing the nerfing showed an extremely poor understanding of what actually needed nerfing about wizards and so the only meaningful spell that got hit in a significant way was Polymorph. It was a pretty good nerf to Polymorph, but that's one hit and a lot of misses. Beyond that they nerfed some spells that didn't need nerfing, didn't nerf a lot of spells that needed it, and nerfed other spells in essentially meaningless ways.