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FWIW, the O2's UMA let it handle far more textures than almost any other contemporary system with reasonable performance.

Most other SGIs had single or low double-digit megabytes of texture memory, whereas the O2 could host one gigabyte of unified memory and use a huge chunk of that for textures.

UMA was never about performance and it still isn't. Spark is slower than a 5090.

did they learn why? were there other gains?

O2 GPU was slower than other SGI options at the time, however it could use hilariously larger pool of memory without copying, which meant that O2 could use approaches that were punishingly hard (very tight transfer loops) or impossible (huge textures that couldn't be virtualized due to needing whole texture).

That was because unlike other GPUs at the time, O2's didn't have dedicated memory but shared the memory with CPU - way slower, but zero copies and bigger.

Arguably early home computers and workstations also used "unified memory" :D