The worst part about arena (at least on console) is definitely the troop selection. There is a high degree of variance in both relative and synergistic power for cards even within the same rarity. For example, Rock Worm and Fortress Gate are polar opposites on the power spectrum, but both are common rarity and both can be options in the arena.
Across the other rarities, the gap gets even more pronounced. For example, Frost Giant is almost always a top-tier pick. He has 1:1 boost ratio with skulls on the board AND the card doesn’t remove skulls. Considering the low health/armor of level 10 cards, Frost Giant makes quick work of targets. Now lets use Black Beast as an example. Black Beast is an awful card to pick in the arena. It reduces your mana spread potential, indirectly increases the damage your team takes from spread/AoE damage (since now there are fewer targets), and doesn’t actually provide you with any more total stat points than you had previously.
Another issue, in my opinion, is the power scale for true damage and being stuck with level 10 cards. I often find myself in situations where the opponent’s true damage can 1-shot at least 1 ally on my team before I can even make use them. Once one ally/enemy falls, it generally causes a snowball effect and the rest are soon to follow (another reason why Frost Giant is so useful).
This probably belongs in a more detailed post, but there are great number of issues at play in the arena. If you get some poor draws as options in each tier, getting defeated with the “lucky” constant mana/skull drops from the opponent is a slap in the face.
The update hasn’t hit XB1 still, so my comments are all based on the pre-1.07 console update, but I’ll also note that the impending forced weapon selection sounds like another awful change in a mode that is in dire need of some TLC.