Ok, so I have recently been playing an alt account and have some thoughts on this.
UDK is a very defensive champion, and comes with very high level regen gear (6* legendary boots) that a player can't farm until about 90 days into the game. Yes, that skews arena a lot. For example with Deliana, it is easy to beat her just with a slightly better Deliana. With UDK this is not the case.
UDK is not unbeatable. However, he does require very optimized builds to beat with starter champs. Keep in mind, new players don't farm campaign optimally. They get misled by the artifact challenges (atk% gauntlets), they don't rush towards 60 with their starter. They don't ignore crap rares. etc, etc.
So rather than a gear check, it's a game knowledge check. And when veterans tell newbies that their problems aren't real because they could have avoided them by spending dozens of hours reading/watching tutorials and then playing the game optimally from level 1 while navigating a slew of information, that shows a serious lack of empathy.
For reference, it was only about 30 days into the game (on my first account) that I even started reading what the champions on the opposing arena teams did. A good game shouldn't encourage that kind of sloppy play, but it also shouldn't punish it as heavily. Rather than a gentle onboarding slope, raid faces you with a cliff at the start.
So yes, Plarium did make a mistake when releasing UDK. "Is he technically beatable?" is asking the wrong question.