I have one but have yet to event in it (more on that in a sec).
I have an A4 and am on the bigger side for a woman (5’9, 150, bigger shoulder/ribcage, smaller hips). I think mine is on the small side for me–it’s definitely an effort to get on and requires a helper. I didn’t realize there was a size 5, perhaps that would be better.
I don’t find the weight that bad, but it does ride up very high, above my jaw (possible fit/size issue), which feels wrong, probably more so because it is so solid. I’m not sure if that would be uncomfortable if I fell on my side.
Personally, I think the biggest challenge to its market success is that it is designed for a specific and rare (though catastrophic) fall scenario–the rotational/crush injury fall. That mostly applies to upper level riders, and probably even they are trying to convince themselves it won’t be them.
Most vests are about cushion, but so are most falls. If you get bucked off your 4 year old, falling with a metal cage around you is probably going to be less helpful than that useless-in-a-rotation piece of foam vest. The Exo’s value is really for high level xc, which isn’t what most vest purchasers are doing.
I bought mine after a bad fall on a prelim xc course (vest wouldn’t have helped in this scenario–was thrown clear). But since then mostly rode a young horse, wore a regular vest (the Outlyne) for trails, jump schooling, even N/T events. I think for me, it was dealing with the fit issues vs the types of falls I was expecting to have that dictated it. I still want to get more comfortable in the Exo, and plan to use it if/when I do higher levels. But I get why “average” eventers who are spending most of their vest time not doing high level xc don’t select it.
ETA: There is also something a little more comfortable psychologically about buying a foam or air vest, and saying “this will make falling more comfortable” than buying an Exo and saying “this will prevent me from getting crushed to death when my horse falls on me.” There is a lot of reality going into that purchase which may be a factor.