When you compare it to troll form, umber has 2 more ac, the same ab and str, 1d8 vs troll's 1d6 dmg, and way less hp. Umber gets spot bonuses and trolls gets regeneration. So its pretty much 2 ac in combat vs regen.
I think it would be much more useful to give the form a bit of love so its comparable to trolls. For instance, upping its APR to 2 would make it solid I think.
Or just give poly overall some lovin. Like +1 ab/ac with sf trans and +2 ab/ac with gsf. To make those poly chars a little more viable :p
Once upon a time, Umber Hulk form had True Seeing, and that was arguably what made the form balanced with the others. As I recall, it was removed for being overpowered, and I guess nothing got put in instead.