It's very easy to see what the big problems are and why barely anybody is actually enjoying the current state of Feast 4v4 (Ranked), but this somehow has slipped past the development/feedback team. The points I make here are from the experience in over 5000 matches played in total in Feast 4v4 alone. I will only be talking about the Combat System here.
1) The new CC system just does not work.
The main reason is because a DPS can be killed from 100% health in around 3 seconds purely through normal GCD w/ OGCD-spammy damage from both enemy DPS. A DRK can prevent all GCD heals for around 7 seconds every 10 seconds (15 with the resist trait on). A PLD for 5 seconds. The math just doesn't add up here, especially with the horrible amount of lag when a heal/shield is applied to a teammate which amplifies the issue.
There is too much CC happening in too short of a time, creating a hugely frustrating experience for any healer in queue. If not addressed, there won't be any healers left in queue at all.
2) Additional Actions
The above mentioned small casting window every 15 seconds actually makes this CC system much, much worse than the previous one in Heavensward. Yes there are traits such as Purify & Concentrate but these just aren't enough to deal with this issue. Muse is a must-have on every single healer. Sprint is never used by anyone other than Melee and even then, sometimes it's better to just take Safeguard and Recuperate.
These "Additional Actions" are some so limited and some so crucial to each Job that they should actually be available without taking up 1 of 2 Additional Actions. Perhaps move Muse to every single healers hotbar? Perhaps give everyone Sprint back and have Bolt as an "extra" Sprint?
3) Redundant Main Job Actions
There are some Job Actions some Jobs have that just miss the point entirely.
Astrologian has Deorbit instead of an actual healing ability, althrough Deorbit is a really cool skill if you can pull it off properly. 99% of the time, it's incorrectly used and when it is correctly used, it's gonna be Frontlines. Why not have Deorbit as an Additional Skill for all healers and give Astrologian something truly useful in a typical 4v4 Combat Scenario?
Warrior has extra damage instead of a CC. That "extra damage" is largely useless as the healer can not longer be interrupted through it. The CC itself can outright win matches and make the enemy healer useless during crucial moments for your DPS. The extra damage just doesn't make up for the lack of CC.
Red Mage has its whole burst locked behind the GCD. Samurai has a 3k potency Action with a Cast Time of 2s, meanwhile MNK can burst for 7k in 2 seconds or so. Samurai needs attention, the extra mobility and weak Chiten barely makes up for the lack of burst, considering the current 13k+ health pools.
4) The lack of OGCD's.
The reason why this is a problem is simple:
Less Buttons = Each Button has to has to have a Higher Potency.
Why is this a problem? It's because there's no longer any trickle/peel/damage. In Heavensward, there would be 3 main skill brackets of DPS players.
Low Damage/No Mechanical knowledge (Completely new, Bronze/Unranked)
Average Damage/Some Mechanical knowledge (Silver to Diamond)
High Consistent + Burst Damage/Full Mechanical knowledge (Diamond)
Now, there's no "Average" players left. Either players output High Damage, or not enough damage to take advantage of the enemy healer being CC'd by their Tank, which gives the healer too much room to heal. Nothing will die, even in culling 2+. It creates a very strong imbalance of players. There's no longer any "solo" potential as a DPS to make up for one other DPS's shortcomings.
The best way to fix this is to introduce more OGCD's, thus lowering the potencies of each other Job Action itself, while slightly increasing the power level of all Jobs. It gives all Jobs more options for them to deal with each situation more flexibly, instead of Spamming attacks all on the same player and hoping the Tank CC's the healer at the right time.
5) Tanks are not longer "Tanks"
Healers have to heal. DPS have to output damage. Tanks are now kill-enablers with slightly higher defense and some party utility, but no way to defend themselves properly.
Taking 4+ Stacks as a Tank is sure suicide - there's no way to outheal the incoming damage, even with the Tank stance on. At 2 Stacks without a Tank Stance, your healer will lose all their MP just on keeping you alive. This creates a Meta where everyone should take 1 stack, which worsens the above mentioned problems by a good margin.
End Notes
There are a few other issues but these are the main ones. Maybe parts of it don't add up correctly but its a long post I made in around 20m or so, eventually I'll edit any inaccuracies out.
If you disagree, feel free to explain why.