Great review Insomniac! Really gets us another perspective on another potentially great glove.
I've seen these gloves before but never dived them. I've done some research into what they're made of, but haven't ever been able to establish their exact composition since the term "rubber" is somewhat ubiquitous and refers to practically any kind of polymer (latex, nitrile, PVC, PU, etc).
My gut says they're latex-based since the company that makes them appears to heavily invested in latex production. One reviewer even mentioned that they can be repaired with a bicycle inner tube tire patch kit, which supports the notion that they're latex. It's possible that they're even 100% latex, and are probably not a true natural rubber coming from a rubber tree. If that's the case, then I expect them to wear not quite as well as PVC gloves like the 620 and 660, and to get "gooey" over time like a latex wrist seal does... But even while they're intact, to not offer the same puncture and abrasion resistance that the nitrile 720s or even the PVC gloves like the 620 and 660 offer.
The fact that the ring can seal against the inside of the glove tells me that the glove must be of the unlined variety... Meaning that it'd be cold and clammy without glove liners. Therefore, glove liners are kinda a prerequisite for use, which adds to the thickness of the glove and reduces dexterity. One of the advantages to the 620s, 660s, and 720s is that they have a low-pile fabric liner that makes going undergloveless quite comfortable, adding to dexterity and tactile feel if the water's not too cold. The 620s and 660s are both cotton-lined (think "T-shirt") while the 720s are lined with an acrylic material (think "wicking T-shirt"). The acrylic liner of the 720s does a nice job of wicking sweat, staying clean, and drying fast without giving a cold, clammy feel of an unlined glove. See the long video I posted above where I show the clean, white glove lining inside the 720s.
FWIW, the SHOWA 460 is essentially a 620 with a "fuzzy" insulated liner, although they may not work for everyone since it has a rather short cuff... And the "fuzzy" liners can sometimes cause leaking issues with the gloveside rings on dryglove rings. There's also a seam at the base of the glove, which complicates installation and reduces stretch... And if a "fuzzy" glove gets wet, it takes forever to dry. I generally don't recommend "fuzzy lined" gloves in general... But I digress...
Another reviewer mentioned that "rubber" gloves you're talking about were slippery and thick, offering little in terms of grip and dexterity. You mentioned that you felt little difference between them and the 620s, even when dived side-by-side... Which tells me that if there's a problem with grip and thickness, it can't be too bad. The thickness has an advantage, by the way, in terms of helping to improve durability - so it's not wholly a disadvantage.
...But the fact that they can be used without the glove rings (eg in an emergency) and/or installed "the easy way" with the inside of the glove sealing - makes them unique and I imagine in some cases the best choice.
At any rate - I really hope you'll get a set of the 720s and compare them to the gloves you have. I suspect you'll find them to be stretchy like latex gloves (unlike PVC 620s and 660s) while offering you more toughness and durability than any glove you have reviewed here thus far. They also stay cleaner and are naturally warmer than the gloves you've mentioned, which may promote you to go undergloveless... Which always increases dexterity and tactile feel. They DO require a seal against the OUTSIDE of the glove since there is a low-pile liner in them (like the 620s and 660s), but their additional stretch makes them a LOT easier to stretch onto the ring. Don't be fooled by their thinner, stretchier material - they're more durable than the 620s and 660s. Nitrile is simply a tougher material than PVC.
Standing by for your review... Something tells me that if you're accustomed to an underglove, a PVC glove, and then in some cases a "rubber" glove over that, you're going to find these 720s amazingly liberating and natural-feeling.