There's also somewhat of a game balance issue.

If my flying HSSer could make his attacks from outdoor blocking terrain, then your PC would have no effect.

Indeed. filler

Not completely off the top of your head, as we do have this lovely power (that illustrates your point):

SUCTION: When an opposing character hits Ultimate Clayface with a close combat attack, after the attack resolves, the opposing character rolls a d6. On a result of 1-3, that character immediately ends its action.

Sure he has plasticity his entire dial, but if that special power gets outwitted that would stop a hypersonic character.

Harpua's point was what I was going to say earlier.