Why each class is in its tier: 2019 update!

[Anthrowhale]

I'm just kinda skeptical that polymorph covers all the ground of the special qualities of all aberration forms. It's a lot of ground.

Going through your guide, you call out:
L8: Nilshai: Extra standard action/round --- excellent combat form for dispensing spells.
L8: Dharculus: Ethereal plane based --- excellent stealth.
L9: Will'O Wisp: Immunity to Magic, Natural Invisibility --- excellent immunities.

Polymorph grants Thoon Elder Brain Dual actions, 12-headed hydra attacks, Wartroll's dazing blow, or the plant type's basket of immunities. Polymorph Any Object broadens the set of forms substantially at level 15.

And, when Shapechange hits it obsoletes everything else.

Overall, I'd give an edge to Enhanced Aberration Wildshape although it's fairly close.

Transformation domain cleric is pretty specific and even somewhat obscure. What percentage of spontaneous clerics are going to choose it? Of that percentage, how many also take DMM: Persist and don't have better spells to use with it? Because every spontaneous druid starts with wild shape as a baseline. If we assume power level is ranked based on a weighted average of what we expect lots of different builds to look like—and it is—I'd bet that the Transformation domain's impact on its respective class is much lower than wild shape's. Plus, druid also gets the animal companion, which is another powerhouse class feature.

To the extent that Eggynack's guide has made Druid's more powerful by making many obscure things well known, that seems reasonable. I haven't seen a similar treatment for clerics yet. When/if someone does that, maybe usage patterns shift.

The transformation domain however is notably only an example. There's also things like Initiate of Mystra, demonic domain, spell domain, mother cyst, travel devotion, and more general persistomancy which are also generally on the table, with several of these combinable.