@tight harbor requested that we give feedback on some arena-related topics a few days back, so I figured I'd type downs some of the things I've noticed:
Basic synergies are far too overpowered on higher waves when you're fighting 11 illuvials all getting the full bonuses. Specifically 5+ Fighter/Bulwark/Psion, although Psion might me more of an issue with decks just not having anything to counter the enemy time fast enough (Not enough Rogues or Psions, or not enough Damage reduction or Healing available)
Some Composite synergies are really underwhelming, or they come on units that are just far too slow to Omega to have an impact, if they're Omega-based, or they're linked to abilities or effects that just doesn't trigger enough to make a difference in the fights.
Examples:
Aegis: 400 shield and 40% of the unit's resistances give to the nearest unit. The available Aegis units have very low resists, and having bonuses doesn't seem to influence the amount given.
Predator: 25% Crit Chance, 40% Crit Amp, Execute enemies below 8% health. The last ability just doesn't feel impactful in any situation where you're not already winning.
Revenant: 15% Crit Chance, 400 Shields on allies within 25 hex on Takedown. The shield range is a bit small to be impactful considering it's a Rogue class. it also only lasts 5 seconds, which is usually wasted after a takedown if there's running distance to other mobs (which frequently happens given it's a rogue class).
Vanguard: 25% Crit Chance, Takes 50% less damage and Taunts enemies for 4 sec on Takedown. If there's no other enemies around the secondary ability is useless. If there's too many mobs the damage reduction doesn't help as it's being focused by every mob in the area.
Granite: +15-35 Grit/Resolve and Invulnerable for 4-6s on battle start. Generally. Units don't die within the first 6 seconds and after that the remaining bonus isn't feeling impactful.
Inferno: Units first Omega superheats the air and deals damage over time for the rest of the fight. Sounds interesting, but the damage is just too low to be noticeable.
Magma: Similar to Inferno, only it's always looking far more effective when used by the AI. When players use it it just never seems to do any damage over time.
Wildfire: Same as above, the damage over time just isn't noticeable, neither is the heals.
Spore: 15 Dodge, Heal 50hp and reduce energy effectiveness when dodging. Even with 50%+ Dodge I never see it triggering often enough to be worth using.
Tempest: 18 Dodge, Steal 2 energy when dodging. Same as Spore, it just doesn't trigger enough to be worth using.