A speed boon makes him very fast for an armoured unit. I’ve built a +spd -Hp player phase Zelgius and he doesn’t need bold fighter to double his usual opponents.
I have had a few +spd -atk enemy phase units and they work fine. The spd helps bulk tremendoysly and its easy to boost atk while specials ad dmg. Sure its not perfect but the spd boon helps faaar mre then the bane hurts.