After playing with the alpha for a few days I've noticed 1 bug so far, probably very minor though.
1) For some reason after the 2nd boss in PoS hc two of my party members health bars went empty. During the fight 1 of my party died, and was res'd after, however both him and the healer were showing no green health bar, yet it did say 100% ( bar was visible, just empty).
2) Layout. I understand the need to change how the Nui display of party and focus targets caused many people problems. Focus going over the mini map can be some what annoying. Just without being able to turn off (i know it's by design atm) portaits, which imo aren't needed, you take up too much room and end up compressing bars/data to the point it is for me unclear.
I've not had the opportunity to try this in a raid situation but I get the distinct feelings it's going to be somewhat chaotic to track anything.
3) It seems to run smoothly and in the 5 mans i've tried I've not had any performance issues at all.
Keep in mind that the portrait area is used for other important things, too, like party role, raid group number, raid target icons, pet happiness, combat and rest indicators, ready checks, combat feedback and a slew of other features that we do not want to be covering up the health and power bars with... it's just that none of those features are currently being displayed on the unit frames. The portrait is really a "filler" for that space when not being used for all of the other purposes set aside for it.
What people don't get is that I am, ultimately, an artist at heart. My brush has two colors, 1 and 0, and my canvas is made of silicon.