Chips are often processed in the 400C+ range, so expect fine-pitch devices to fail due to uncontrolled diffusion around there. Not to mention oxidation of metals (RIP aluminum film?), silicon oxide growth, and other metallurgical problems (say for copper interconnect devices?).
Probably, a lot of chips subject to modest pyrolysis (enough to char the package) can still be used if rebonded, and possibly if re-metallized as well (which, mind, would be a pretty heroic effort, if the top metal layer is just trashed, and you don't know what the chip was originally).
Chips are so cheap that it's very much worthwhile to consider even mere loss of legs/pins/pads a total loss of the chip. It's still perfectly fine inside, it's just not nearly worth getting at.
It can be justified if the chip contains data worth the while: Flash chips, or stacked CPUs for instance. For which, you can find videos of repairs done by grinding into the package, locating a particular trace or pad in the interposer, and running a bodgewire to the PCB.
Tim