I actually quite enjoyed Dreadwyrm Trance in Heavensward. I feel like it gave the job a nice cadence and build up to this really explosive damage phase that really doesn't exist anymore with Stormblood. There are two big reasons why.
The first is that they locked Aetherflow out of Trance and made Aetherflow and Aethertrail mutually exclusive. Part of what made Trance feel so powerful in Heavensward was getting to go all-in on your high damage spells. You got massively reduced cost Ruin III's and juicy Festers. They've since changed it so you can never Fester in Trance and Ruin III was heavily nerfed to make it a basic Ruin upgrade. You always cast Ruin III now, it's your filler spell. Having a slightly stronger filler doesn't make Trance feel as powerful as it used to.
The second is the introduction of Demi-Bahamut. Trances feel less like their own thing now and just a stepping stone to the real high power phase of Demi-Bahamut, but since he takes so long to get to you're left with several low phase and two mediocre phases before you cash in on that high. In addition to that, while you can't Fester in Trance anymore, you can Fester while Demi-Bahamut is out, which further shifts the power balance in his favor.
This to me makes the job feel kind of lackluster in that you need a lot of buildup over several minutes just to get one really good burst phase.
I just wanted to highlight this since it also plays into one of my complaints about how the current Summoner is a mess of shallow disconnected themes. The old Contagion actually had a specific tie-in to your kit, and was a link between your pet and your DoTs. It wasn't amazing, but it was there. The lag was also less of a problem because she had at least 15 seconds to tag your DoTs and then it was good to go. Not like now where a 5 second delay means you've missed part of the trick window or just want to move on with your rotation but are stuck waiting for her bland magic vuln to apply.
I honestly don't like the new Contagion because of those reasons. Yes, it is undeniably stronger, but it lost all of the Summoner-specific flavor it had before. Now it's just... there. It does everything for every caster job, except for auto-attacks.
I mentioned it in one of my previous threads, but neglected to bring it up here for the sake of brevity. I honestly think Bahamut would have been more interesting if he actually responded depending on which Aetherflow abilities you use, instead of just casting a generic Wyrmwave for everything. If I remember right it was Flare Breath for Painflare and Bane, and Flatten for Fester and Energy Drain. Do you think that would have given him at least more personality depending on the situation? Would that be a more engaging way to tie demi-summons to the core aetherflow mechanic?
Edit:
I mentioned it up above but didn't really elaborate, so people new to the game or just Summoner in general might not know what I'm talking about in regards to Heavensward Summoner. I'll post that here just to be clear.
In Heavensward, your filler spell was Ruin. It had a potency of 80. Ruin III had a potency of 200 but cost around triple the MP as a single Ruin. While Dreadwyrm Trance still gave the same 10% buff, it also reduced the cost of Ruin III down to a normal Ruin. You could not just spam Ruin III the entire fight or you would run out of mana. This meant you had to use the 80 potency Ruin as filler and you absolutely noticed the shift in Dreadwyrm Trance to the heavily discounted Ruin III's.
With Stormblood they buffed Ruin to 100 and eventually nerfed Ruin III to 120 to make it a straight Ruin upgrade. This means you are always using your filler spell and there is no consistent major spike in damage like there was back in Heavensward. Ruin IV kind of fills the gap, but those are random and subject to chance. You could get 4 back to back in a Trance. You could also get none. They kind of re-added this mechanic back in with Tri-Bind, but personally it doesn't remotely feel the same since AoE isn't really a thing in raids, trials, or most fights in alliances. It is great for dungeons though.
This is why I feel like Trance got a lot less valuable as a 'burst phase,' because they normalized a lot of the damage.