I'm just interested in evaluating the diving, not the topside/ restaurants/ nightlife or budget considerations.
Understood, but I'll touch on some of that anyway, for the benefit of others who may see this thread doing their own trip research. I had one dedicated dive trip to Cozumel and some cruise ship visits. I'll address it as a Caribbean destination, not comparing it to Raja Ampat.
1.) Warm water (but far enough north to get a little chilly in late winter, at least; this ain't the ABC islands (e.g.: Bonaire).
2.) Excellent viz.
3.) Varied topography, particularly if you do some Palancar sites.
4.) It is drift diving. Some really like that, some (like me) can struggle with it a bit. There seems to be kind of a knack to it.
5.) I thought the reefs were generally good overall and the sea life pleasing - more 'big stuff' than I saw in Bonaire.
6.) Depending on where you stay or go, there's some shore diving. That's not the main draw, but it exists.
7.) If you have non-divers along, it's a (very) popular cruise ship stop, so it'd got a number of topside excursions.
8.) You can stay in town fairly cheaply and eat out well at various places - a thread on where to dine out in Cozumel can dwarf non-specific threads for many destinations. Good food at a good price is a selling point.
9.) You can stay at A.I. resorts to the south if you want all inclusive and short rides to the reef.
10.) If you have high gas consumption and would like a 120-cf steel tank, there are a few operations that carry those (e.g.: Aldora, Living Underwater, at least one other I've heard of before).
11.) A wide range of budget niche offerings. Aldora has a rep. as being a nice, more expensive op. with the big 120-cf steel tanks; Dive With Martin has a rep. as a fine budget option that, last I heard, I think included rental gear with their package rate?
It is a destination where the diving occasionally gets 'blown out' for a day or so, and that's riskier at some times of year. If you can't stand seeing cruise ships or passengers, or don't like 'touristy' destinations, that could be a problem.
Keep in mind, picking a dive destination isn't always about picking the 'best,' whatever you think that is. Sometimes it's about rounding out your experience. I consider Cozumel, Bonaire, Turks & Caicos, the Caymans, Key Largo and the Galapagos all distinct from each other for various reasons, but I'm glad I was blessed to dive in all of them. Cozumel is special enough that if it's convenient for you and you like to dive a range of places, it merits a visit.