*Membership spots not really limited!
Any chance you are on a slow internet link. I agree with Vader's comments about Flixster, but VUDU can definitely play in HD or HDX. (I use the PS3 app) However, if the app discovers your speed is too slow it will negotiate downward to SD. Only catch though is that when I have seen this happen the VUDU app gives you a big popup message about what it is doing, and judging by your comment you haven't seen such a message...
I know my internet speed is just fine. I only tried the iPad app so far. I'll try the Xbox one when I get home. I know I didn't like UV for a reason.