Reused Boss models/skeletons you've noticed.

[Nomuno]

Ultimates but apparently that's still too much for the devs

about as "too much" as it takes for you to stop whining

[CKNovel]

I don't mind this in general, but there are definitely a few major cases where you really would have figured they would have put in the extra time/effort. For me the biggest one was the major armoured angel enemy from the Shadowbringers trailer. It's an extremely cool looking design and fit ShB perfectly but when I found out it was actually just a reused boss from a hard mode dungeon (which I hadn't done prior to seeing the trailer) I was really disappointed.

Kind of liked the Ampador reference.
It's also very usefull lorewise to show strong similarities between Ampador and the Sin eaters. I forgot what exactly, but in-game, the guardians design came from someone who had a vision about the sin eaters.
I guess because you did it the opposite way, it didn't had the same impact, but when ShB trailer was revealed, it screamed "Light", "White Magic" and such.

However they admitted they were limited and wished to model much more different Sin eaters. With a certain varus going wild around the globe, I guess we'll have to expect the same for Endwalker.
Sometime they can be imaginative like Construct-8 being reused as the "Golem with a bucket on its head" in Malikha's well.

I dont believe reused assets is a big deal when it comes to mini-bosses as long as they introduce different mechanics or a new way to interpret them. The recent Matoya's relic first miniboss shares its model with the first Sirensong Sea miniboss. But when it comes to weapon model used 3 times within the same expansion, I share the disappointment.

[Steelbreaker25]

Tons and tons. More than you might think, really. One of the most recurring ones I see is the Diresaur/Giruveganaus model--aside from being used as a boss in Stone Vigil Hard and mobs in the Churning Mists, among other places, the skeleton is also used as the less spikey dinosaur-like boss from Sohm Al Hard; I think it's called the Gowrow, and is also used as a SB S-rank hunt and the second fight in DR. Not only that, the skeleton and animations are also shared with the manticore mobs, such as the Garlok S-rank, the boss from Halatali Hard, and the sin eater variant encountered in Mt. Gulg; and the skeleton is used for Byblos and Apanda from Gubal Library.

I'm also pretty sure the second set of stone statue adds from E12N share their model with the Glasya Labolas from Syrcus Tower. That model is somewhat uncommon though.

But there are so many reused and repurposed models and skeletons in the game.

[Vespar]

I'm okay with them reusing some lesser known bosses. One that comes to mind is Penthisilea from Eureka and Scathach from Dun Scaith.
I'd actually love a figure of Penny. I just loved her design. I'd kill for those boots SE. Make it happen!

[CKNovel]

I'm okay with them reusing some lesser known bosses. One that comes to mind is Penthisilea from Eureka and Scathach from Dun Scaith.
I'd actually love a figure of Penny. I just loved her design. I'd kill for those boots SE. Make it happen!

Ah yes, lewdzuzu.
I'm pretty sure there is a quite recent boss reusing that skeleton but I can't put my finger on it...

[MicahZerrshia]

Ah yes, lewdzuzu.
I'm pretty sure there is a quite recent boss reusing that skeleton but I can't put my finger on it...

The necromancer boss in the Heroes Gauntlet uses this same skeleton but with a cape and staff instead of the wings

[Eloah]

Ah yes, lewdzuzu.
I'm pretty sure there is a quite recent boss reusing that skeleton but I can't put my finger on it...

Also the final boss from Mt Gulg, Forgiven Obscenity, uses the same skeleton.

[Ultimatecalibur]

Ah yes, lewdzuzu.
I'm pretty sure there is a quite recent boss reusing that skeleton but I can't put my finger on it...

Forgiven Obscenity is likely the one you were thinking of. If not her Oracle of Darkness is another candidate. Spectral Necromancer uses the Succubus skeleton and Leannan Sith used the Siren skeleton.

You know what I'm now starting to suspect that the Succubus, Siren, Scathach and Shiva skeletons are the same "floating female" skeleton just with varying accessories and different idle and attack animations used. Would make sense to do it that way as it would make things more interchangeable.

[Erendis]

The monster WoL and twins fight in EW trailer is same as Amdapor keep last boss.

[Jirah]

I swear the Anata and those Siren chicks in Sastssha HM are the same