I'd like to draw attention to the most important change that summoner received in Shadowbringers - the removal of aetherflow - and why this change has serious ramifications on how the job will be played in battle content going forward.
As you know, aetherflow has a 1 minute cooldown and grants 3 aetherflow charges. Spending them is how you activate dreadwyrm trance (and in turn gain bahamut charges). In order to build up to bahamut, it is important to use them every minute before you hit aetherflow again.
In 5.0, dreadwyrm trance will now be the button you want to hit on schedule every minute, it still grants bahamut/phoenix charges after use, and is now a simple 1 minute cooldown not requiring aetherflow charges to be spent to unlock it. This move from aetherflow to dreadywrm trance as the bottleneck of the rotation dramatically changes for the worse how downtime affects summoners.
The strength of aetherflow lies in the fact that activating it to gain charges is seperate from spending those charges. Even if aetherflow coincided with a break in combat, you could press it anyway to start the cooldown and wait till the enemy returned to spend them. Regardless of the length of downtime, the overall flow of the rotation was unchanged as the only bottleneck was aetherflow, and you were able to keep its cooldown cycling on schedule.
Dreadwyrm trance is different as unlike granting you 'ammo' like aetherflow, it activates a buff that starts immediately. As the cooldown will not begin until you actually use it, this means that if you run into the situation above, you will be forced to either use it now and waste it, or wait till combat restarts. Firebird trance is even worse as 'summon phoenix' is combined with it too. Even if you are forced to waste dreadwyrm trance, you can at least still summon bahamut at a different time when convenient, but not with phoenix.
Both cases result in lost damage. Either immediately, or by delaying your demi summoning cycle permanently for the rest of the fight, reducing the total number of demis you can summon. This will happen in all settings, be it raids, trials, or dungeons. Even solo and outdoor content.
Conclusion:
The nature of aetherflow meant that till now, summoner has been well equipped to adapt to downtime. It rewards knowledge of the game and gives you the agency to plan ahead. With dreadwyrm trance taking its place, we are losing all these advantages. This is very disappointing in an expansion where the dev team has started to take downtime into account in job design. New additions like the charge system allow cooldowns to 'overflow' without penalty, and jobs like monk and black mage are getting improved tools to carry over resources through downtime.
Summoner in contrast is going backwards in design several years. Instead of being able to bank resources and sync up burst phases with opportune moments of an encounter, our trances and demi summons are going to turn into 'use it or lose it' static cooldowns like the infamous wildfire.