Well the most obvious would be the visual. If its a world where looking like The Thing would be a problem that could be a real concern.
Otherwise..its mostly theoretical. If you took no flaws or disadvantages technically their would be no effects.
But you could take some cool effects.
He could take a flaw for no sense of touch..or no sense of smell...or something like that.
You could also stack a power that made him immune to actual wound effects, but take a flaw that causes him not to be aware of how hurt he is (When hes in golem form, you track his damage.
The game is flexible enough that if you were to take no flaws the visual would be the only detriment though.
Odd question here...
What are some specific drawbacks to having a transformation type power?
Like a 2HD Granite form power that just makes you look like a golem, then attached strength, HAR, and Extra Tough? Point wise it is clearly the best way to go however i am looking for specific situations where this would be a detriment.