According to Zwiftinsider it changes the rolling resistance to a road tire on tarmac:
When activated, the Steamroller makes whatever surface you’re riding roll as fast as a road tire on pavement. So if you’re using road wheels, it is only effective when deployed on a surface other than pavement.
Hence it would equal to ~90W savings (MTB+dirt compared to road+pavement). In comparison, it translated to 190W savings as used yesterday - for which the discussion is that it had unbalancing effects. Or would be close to the 100W savings a MTB has on a road bike when riding dirt. I think the unpredictability (can be deployed by riders anywhere), 90W savings, plus everyone having the same PU (rather than chance) could make for quite an interesting tactical race.