Locart GrapheneOS developers don't like it, but that doesn't make it a bad system since targeted and attacked people receive a notification that their devices are compromised.
As far as I know, both PAC and MTE are enabled on GrapheneOS. And both will tell you if there was a memory safety issue detected. Neither will be able to tell whether the memory safety violation was because of an attack or just an innocent bug.
From my limited understanding of PAC and MTE, MTE detects memory safety violations before they happen, whereas PAC only detects signs of maliciously overwritten memory after it has already happened. MTE is also able to detect more memory safety violations than PAC, but they work in different ways, so having both might still be beneficial.
Locart I read a discussion between security researchers a few days ago where they said, in short, that the MTE would significantly increase the number of CVEs and push people to take Apple products.
If people move away from a product because more security vulnerabilities gets found and fixed, they are stupid.