I was excited to try out Cassidy since they're the first support orientated summon, but their performance imo has been underwhelming. Unlike companions, we do not have the option to control summons manually so we must rely on the summon's AI to get things done. The AI does a decent job for most summons since dealing damage is not too complicated, but when it comes to Cassidy's use of their two supportive powers, Holy Ward and Resplendent Aria, the AI fumbles quite a bit.
When summoned, Cassidy will use Holy Ward on an ally with the least amount of health and will maintain that stance on that ally indefinitely unless interrupted. The issue I've had is when they use Holy Ward on someone that doesn't need a HoT like Cait or Etheryn compared to your party's tank who was passed over because they have higher health. Another issue I've had is that Holy Ward is a good stance to reset but the AI doesn't seem programmed to do stance resetting. This is a shame because it would be a much better use of their turns when Arias of Waves is on cooldown than making basic attacks for unimpressive damage. Resplendent Aria is a great power, but Cassidy seems just as likely to use it on an ally who used a 2 Recharge power as they would on an ally who used a 4 recharge power.
Overall I'm let down by this summon since on paper they seem great but they're just not living up to their potential. For them, I really wish can control summons manually especially if more support summons are planned for the future.
Also, a general gripe I've had about summons is that after getting summoned they get put at the end of the turn order before they can take action. Like, compare a summoner who rolled last vs first. Last is already at the end of the turn order so the power taking effect at the end is no issue. But for first, having their power take effect at the end is no different than having gone last except now you've let the enemy act before you or twice (depending on which party rolled higher) before you do anything else in the round. It's just generally worse to roll high on initiative as a summoner. The easy fix imo is to have them act immediately after you summon them, then have summons roll for initiative for the following rounds.