> has reverse engineered a bunch of popular Mali ISAs

As a FOSS user and developer, this is already a big problem to me. Although the situation of Nvidia's mobile GPUs (e.g. Tegra) is a bit better than desktop GPUs, but not much better.

Why is it a problem for there to be additional hardware usable with open source drivers? Or is it that it was reverse engineered?

The problem is not reverse engineering, but having to reverse engineer.

i should have clarified that i was asking why Mali is more terrible than Adreno, MediaTek, etc.

AFAIK, all embedded GPU drivers have to be reverse engineered and none are open source.

as you mentioned, it looks like Tegra might have some open-source contrib from the mfg.

The VC4 (RPi GPU) drivers were written by an open source contributor (Eric Anholt) who was employed by Broadcom to do that.

After many many years of petition from the community to even release documentation on the VC4.

Mali has never come close to Adreno. Simple as that.