I'm chiming in here as a new player, around 40 days of excessive gameplay, and with more spending than I like to admit. I am completely fed up with UDK in arena. He is so broken at the level he is at, and as mentioned above he seems to be in a very high % of enemy team comps. For whatever reason, going from 5* to 6* is like an exponential jump in his survivability, and I don't even like to think about the hours of my life wasted trying to take down a 60 UDK after the rest of his team is long-gone.
Many times I have just forfeited the win because he just doesn't die. You can get him down to nothing in health and he can just hover there, seemingly invincible forever. 50 UDK seem to die within a round once down to nothing. But the 60's live on forever taunting you with the smug constant lines of 'too awesome to die' and 'everyone get comfy'. I personally think Plarium created the guy to troll the player base as hard as possible with all his mechanics and his absurd run.
My team that has done the best so far vs UDK teams is 60 Pythion, 60 Ronda with brimstone, 60 Fellhound, and 50 Magnarr. The Ronda and Fellhound have full masteries. Fellhound is fully booked and in 6 pieces of legendary instinct gear and his A1 aoe is one of the few reliable things that I find I can use freely when Ronda doesn't have her A2 blocks on UDK. And yet even when his bar is empty I have yet to see a brimstone proc actually kill him. The games sometimes show UDK with stats in the millions for damage done, damage taken, and healing done. If I didn't have Ronda, I don't know what I would do. I have fought tons of UDKs now and sometimes they die and sometimes they don't, though they almost always get their bar to empty whether it takes a short effort or a long one to get there. So why do they suddenly get so invincible when they are empty bar?
In terms of arena matchups. it seems that in Silver 2, where I have basically been sitting since day 1 in the arena, that most of the available matchups are significantly higher power level than me. Does this mean that I am playing above my level? It seems that at least 2 50s on the enemy team means I will be able to do well vs them. Unless they happen to all be legendaries, which I find happens way too much. Or if one is a 60 UDK. This sure seems like the majority of all of my available matchups. Occasionally I will get a decent refresh where half the games are winnable. But this is the exception rather than the rule. And tag-team is a whole other level of imbalanced. In there I am Bronze 2, and pretty much 80% of my matchup options have 2-4 60s on every team, and it's hard for me to find matchups in either arena version where the enemy team has a power level that isn't significantly higher than mine.
I don't mind the grind, or the learning curve and yet UDK is a massive hindrance to fun and is IMO bad design balance.