The only issue I really have found

The only issue I really have found with Kanon is his lack of 3-turn status negation on his SBB, meaning teams that autobattle might not be fully protected from status if Kanon does not get SBB up every single turn. However, this is easily rectified by battling manually, using Kanon's BB every 3 turns (or more often if a boss strips buffs) to prevent status. Also, I decided that Kanon is better than Ulkina/Rigness usually (not counting elemental weakness and healing source requirement) because on Ulkina/Rigness, you'd normally have to put Drevas or waste a sphere slot with a Refined Gem, however Kanon's ES allows you to put a different sphere on him such as Legwand/Malice Jewel without having to worry about Kanon himself getting cursed/paralysed.

Kanon's UBB however, is something you will probably never see used, unfortunately. In situations where you would actually need the damage mitigation, you'd already have a mitigator in your squad. Furthermore, his Def buff on UBB is only 10% higher than on SBB, and just isn't worth the entire overdrive bar and the additional BC to fill. Doesn't mean that Kanon is a bad unit, of course, just that for UBB you'd probably be better off using someone else's.

It might be worth noting however, that Kanon does not immediately remove status, the status cleanse only activates when Kanon finishes moving up to the front and starts "flashing", so it might be better to put Kanon in the middle of the front row to speed up the status cleanse when you activate his BB/SBB.

Kanon's LS is actually really good at keeping your squishier teammates un-squashed. 30% boost to HP and Def is quite strong on its own, and when put together with Kanon's status cleanse+ huge Def buff on SBB, your whole squad becomes really hard to take down, and this is why Kanon is still a great unit to have in JP till this stage.

Kanon's usage:

With a 22-dropcheck basic attack and an AoE BB, Kanon is certainly usable in Arena, however he is certainly nowhere