It's been a few days, now. I've cleared the new extreme several times, done my Savage reclears, and quite frankly played a disgusting amount of PVP (it's super fun!).
With that in mind, I've had a fair bit of time to chew on this current iteration of Ninja. I was going to leave it a few more days before I made this post, but the surprise maintenance left me with a bit of free time, so here I am.
So, my most crucial and opening point:
Mug is a bloated ability.
What I mean by this is that Mug does too much for one ability, and as a result feels bad to play around. Most abilities do precisely one or two things, and no more -- they deal damage, have a single effect, or deal damage and have a single effect. Mug does three things. It deals damage, provides a party buff, and provides the Ninja with some resource.
This bloatedness means that you essentially need to prioritise which effect is more important, and deal with the fact that the other effect will happen at the same time. In Mug's case, the party buff being on time is more important than the gauge Ninja receives, and thus is it optimal to press Mug on cooldown, regardless of whether or not the Ninja wants (or is able!) to receive the +40 Ninki from the ability.
This is and of itself would hypothetically be fine, if not for the fact that, well -- Ninja's Ninki generation at the moment aligns rather poorly with raid buff windows. You tend to cap your Ninki a few GCDs before raid buffs come up, forcing you to spend them outside of buffs. That, and you cannot enter a Mug window at Ninki higher than 60 without automatically losing some gauge due to Mug giving you +40. In most Mug windows, you're now only able to manage two Bhavacakras, whereas beforehand, you would oftentimes be able to land three. While this might only be 54.25 potency, given Bhavacakra's recent nerf, it's still a decent chunk, and frankly it just feels bad not to be able to build up gauge in the same way other jobs can.
The other issue is that you want to press Mug and Trick Attack more or less at the same time, to maximise the value of stacking buffs together. Consideration was evidently given to this fact, as Mug has a 20s cooldown, allowing you to comfortably activate Trick Attack about a GCD later, and fit the entire duration within Mug. This is all well and good, except for the fact that Ninja has ever so slightly more burst than can fit within Trick -- and you don't really want to start bursting until Trick is active. This means you inevitably land a portion of your burst outside the range of both buffs, which again, feels bad to play around.
So, how do you solve this?
My recommendation would be to move the party buff back onto Trick Attack, and keep the buff at a 1-minute cooldown to align with Ninja's natural burst. This would leave Mug as a simple +40 Ninki gainer, simplifying it's usage and once again allowing for the Ninja to use Mug for that Ninki gain at the most optimal time, rather than on hard cooldown.
This does not have to mean a complete revert to how Trick Attack functioned in 6.08 (though I would greatly enjoy it if that were the case). If Square Enix is dead-set on increasing Ninja's personal damage, a compromise I would swallow and prefer over the current iteration is something similar to Dragoon's Dragon Sight -- a smaller % damage buff to the party, and a larger % damage buff to the Ninja.
Make no mistake, I would much prefer Ninja returns to the comfortable 5% vuln up on a single target on a one-minute cooldown.
Make no mistake, the current iteration of Ninja is as strong or even stronger than it was in 6.08.
Make no mistake, however, that the 6.1 iteration of Ninja simply does not feel good to play.
This is not to say that 6.1 has been entirely miserable for the Ninja. I for one really enjoy seeing the new Trick Attack icon on the enemy -- I think it's much more flavourful and enjoyable to see than the generic vuln-up image. In addition, I think the current PVP iteration of Ninja is incredibly fun (a resetable execute!!).
It's clear that a good amount of thought has been put into the changes in 6.1, such as to not completely change the experience for Ninja players. However, I do not believe that the changes to Ninja were made with the feedback of Ninja players in mind, nor do I feel that the voice of Ninja players has been heard. If anything, it seems that opinions of other jobs on Ninja is taken into consideration more heavily than that of Ninja players (see the removal of prepull Doton).
While Ninja has had a rough start this expansion, with it's undertuning and missteps with forced Raijus drastically lowering the number of players, the 6.08 buffs to Ninja saw it rapidly growing in popularity once again, as it was no longer extremely weak, and felt quite good to play. I see no reason to dramatically change the experience from 6.08, and strongly encourage the developers to walk back this decision to move the party buff onto Mug.