By all objective measures he did a phenomenal job at Nest. He started and sold it for $3.2B. The fact that he failed to continue to be successful post-acquisition, is hardly a "poor" overall record.
I would describe his exit as the only real thing he was successful at. The product is not selling, his employees are all quitting, the acquirer is pissed, the acquisitions he made are all pissed, product development is stalled, Google had to go to plan B with Google Home, ...
Yes, and all of that just shows he isn't perfect. But Tony Fadell will have no problem raising hundreds of millions for his next venture, joining a VC firm, or joining another company as a C-level executive.
Which is interesting because he doesn't really need to do anything else does he? Like you point out, having a $3.2B buyout is pretty much a home run in anybody's book.