I tried a 90w and 120w PicoPSU with a small P200MMX system (PCI video card, PCI NIC, ISA soundcard, 1x CF drive) and found that the system would brown out when lots was going on (benchmarks, lots of graphics activity, etc).
I wanted to test the 'big' PicoPSU; the 160w part, but I never got around to test this.
The 5v supply with them is indeed quite low compared to period AT power supplies. I'm now using the 90w part with a socket 3 system and it works great, but that clearly has a lot lower demand on both 5v and 12v than the socket 7 system.
My collection database and technical wiki:
https://www.target-earth.net