I will preface this by saying I am by no means an expert on the Ninja job, nor someone who has played it for a particularly long, and I do not profess to be. However, I am someone who has enjoyed Ninja for the entire length of time I have played this game, enough that I cleared the recent Savage tier and TEA with it, and I find the changes in 6.1 to be very concerning.
Another thing to note is that I am not concerned about the relative strength of Ninja as a job. How strong or weak a job is in relation to other jobs often comes down to raw numbers, and potency can be easily adjusted. I am mainly concerned about the play feel and identity of Ninja.
In the PLL 70, it was announced that Trick Attack would become a personal buff for the Ninja only, and Mug (120s) would instead become a party buff.
This change feels very sudden, and very unexpected. I think most people would agree that the 6.08 Ninja felt like it was in a relatively good spot -- especially compared to 6.0 release. While there might have been things that could have been tweaked, such as Ninki generation, it did not feel as if it was in need of major changes.
Firstly, Trick Attack is the only remaining 1 minute party damage buff remaining in the game. I understand that there were more buffs on strange/non-2-minute timers in the past, but at least for now, Trick Attack is the only 1 minute buff. When you look into things only at this level, it might be easier to think 'Trick Attack is strange', and have a change such as this be imagined as a result. However, I think most players would agree that Trick Attack is one of Ninja's most defining attacks -- if not the most defining. When I play with other players over voice call, 'when is Trick Attack up?' is something I hear very frequently.
Which brings me to my point: Trick Attack changes the way other players play their jobs when a Ninja is in the party. For example, I have had long discussions with friends of mine who play Reaper and Samurai about how best to optimise their resources to deal as much damage as they can during Trick Attack. It is also rewarding for jobs that have natural 1-minute burst, such as Gunbreaker, to have Trick Attack available.
These changes were supposedly made to increase Ninja's synergy with other jobs, but I argue this change will reduce Ninja's synergy with these jobs. Any job that has 1-minute burst now has no inherent synergy with any other jobs, and any job that needs to spend resources in between 2-minute party buffs now has no particular place to spend them. You could argue it makes these jobs more flexible, I suppose, but that flexibility remains simply by removing the Ninja from the party.