By way of background, the Swarm is an example of what might be called a "distributed multisub system", and is based on an early version of Earl Geddes' ideas on the subject, used with his permission. It consists of four 10" passive subs, driven by one (or optionally two) shelf-mounted amplifiers.
For those unfamiliar with the basic concept, it goes something like this: A subwoofer will inevitably have a nasty room-induced peak-and-dip pattern. You can move the sub or move the listening position and the pattern will change, but it will not go away. When you distribute four small subs asymmetrically around the room, each produces a significantly different peak-and-dip pattern at any listening location in the room. The sum of these four dissimilar peak-and-dip patterns is considerably smoother than any one would be, and that improved smoothness holds up throughout the room. And "smooth bass" = "fast bass", for reasons which I can explain.
Todd Welti's work on the subject of multiple subwoofers for Harman is better known on this forum. Todd examined symmetrical placement strategies, while Earl focused on asymmetrical strategies. By coincidence they were both developing their ideas at exactly the same time, but unknown to one another.
Does it offer any advantages/disadvantage over four separate subs?
No big ones.
The individual subwoofer units are "voiced" with the expectation that they will all be used together, so how they sum is taken into account. Typical "room gain" from boundary reinforcement is also taken into account. As a result, they usually don't need much if anything in the way of EQ.
The four passive subs of the Swarm are normally driven by one amplifier, and optionally by two. Each amplifier includes a single band of parametric EQ, so the EQ capability is limited. If you wanted to equalize each sub individually, you'd be much better off with four separate powered subs.
Can you go with smaller drive units? (E.g. Rhythmik).
Yes. The basic concept can be implemented many different ways. I don't really do custom sizes of my product, but you can assemble your own distributed multi-sub system using whatever subs make sense for you. They need not all be the same size; you can combine one or two big subs with smaller ones. The one caution I would offer is this: You want to roll off the top end of any subs positioned away from the mains with a fairly steep slope, and no higher than 80 Hz. You don't want these subs betraying their locations by passing audible upper bass/lower midrange energy.
I have a very high opinion of Rhythmik subs.
I’ve seen a few intra-thread mentions of this, but any longer-form reactions to this product?
Over on Audiogon there are several threads on the Swarm in particular, and/or on distributed multisub systems, or "Distributed Bass Arrays". You will also find mention of the DEBRA system, a version of the Swarm concept that I designed for James Romeyn, which is essentially the same thing but with a rectangular footprint instead of a square one. Some of the posts there are by people who have been using a distributed multisub system for more than five years. Google "audiogon swarm" and several threads should pop up. Just for the record, there are some rather absolutist statements made which I do not subscribe to. The concept has detractors as well as fans, so I encourage you to read what the detractors have to say.
Robert E. Greene of The Absolute Sound reviewed the Swarm in 2015, and his review is online. I get the impression that reviewers are looked down upon around here, so let me tell you a little bit about Robert: He turned down an offer to be head of the Mathematics department at Tulane University and chose a position at UCLA instead, so that he could work with more people who were at the top in the field. Last I heard he was working on the ninteen-dimensional mathematics which would be the theoretical foundation of whatever comes after String Theory. He is a concert violinist and instructor, and taught Russel Crowe to play the violin for his role in "Master and Commander" (that's actually Russel playing in those scenes). He also runs a Doberman Pinscer rescue.