I just need to look at Inspiring leader feat and then to twilight cleric, and it's enough to see it's completely broken.
1 Action vs. 10 mins activation
6 targets vs. how much you can fit into 30ft radius
1 instance of temp HP per vs up to 10 instances(yes, I know that in most fights it wont be 10 instances, but even 3 or 4 rounds with 1 or max 2 characters utilizing it puts the inspiring leader feat to shame)
One usage per short rest vs. multiple at later levels and/or items
Well there's two things to consider. 1- Inspiring Leader was made 8ish years ago. The current development team may consider it too conservative by today's standards.
2- even though Feats have been compared to "class-agnostic class features" by Mr. Crawford, there's no reason to believe the class feature of a class that has healing/buffing/protection as it's hat can't be miles better than a Feat. Or worse (I'm looking at the final power of the Tiger Totem Barbarian here).
Because, let's face it. The design standards for 5e
have changed. Because of their desire to keep the PHB "evergreen", it has not changed with the times. This is why, as of Xanathar's, we see subclasses have gotten considerably better for some classes- Ranger and Sorcerer still have problems as classes, but it's easy to see that their newer subclasses have been buffed significantly.
The difference in power level between Tasha's and the Sword Coast Adventurer's Guide is obvious. Most of the things that were in SCAG have been redesigned and given back to us in a better format.
While some things have not been consistent (we've seen Unearthed Arcana subclasses be published in weaker/stronger forms, and some ideas never materialized at all, like the original Ranger fix, or a Beastmaster that works), overall, the new direction is pushing design space and power level as compared to the original.
We know that they are going to make significant changes to the core books in 2 years, we've already had a preview of the new design in Mordenkainen's Monsters of the Multiverse.
There are several ideas that could be pushing the game in this new direction that we can only guess at.
The first, and the most cynical, is that Hasbro wants D&D to make MtG money. This will mean a more books, with a faster development cycle and aping MtG's "FIRE" design (Fun, Inviting, Replayable, Exciting). While many Magic diehards gripe at the 'power creep' of the past few years, the product is selling like never before.
The second is that they feel they were too conservative in their original design philosophy. We can point to a few indications of this, such as Crawford's desire to make Backgrounds stronger game elements than "2 proficiencies and a frequently ignored Feature". They want players, and new players, to feel excited about their characters and their abilities, and to encourage people to express themselves creatively (new race paradigm).
The third is that the difficulty of the game may be shifting as well to make it more accessible and easier for the game to be run. We know monsters are going to be slimmed down, especially spellcasting monsters. We can see that short rest mechanics are likely going to be eliminated (which will make short rest mechanics theoretically worse, including Inspiring Leader). This could very well be in response to feedback that many tables aren't running 6-8 encounters per day, but instead are using fewer, more powerful encounters. We don't know, but if the encounters per diem model is changing, then the power of characters has to be adjusted in some way. It could be that Twilight Cleric is a harbinger of things to come.
This could mean that, in the future, that encounters will become more difficult. Monsters may become leaner and meaner. It's hard to say, but the game is evolving. We don't know what it's evolving into, but it's changing.
What seems 'broken' and 'overpowered' now may become the new standard tomorrow. And the power level we are used to, could become obsolete.
It's either that or Twilight and Peace are just massive mistakes made by the makers of the most popular TTRGP in the world, and they will totally admit that and issue power level errata at some point.
I feel however, that we are standing at a crossroads, and some groups are going to have to deal with the fact that future products could very well be pushing the game's boundaries in a direction they aren't comfortable with.