I’m not an expert in these, but since no one is chiming in I can give you a 30,000ft view. They do what they’re supposed to do, but a lot of people would say they introduce more cons than they’re worth. They give you the ability to see mils measured accurately throughout the magnification spectrum, while having an “optimal” reticle thickness throughout. On paper, this solves the reticle being too thin on low powers and/or too thick on high powers.
I guess my thought is that that’s not too big of a problem to need to fix. No one uses precision optics for speed up close, especially now with the proliferation of LPVOs and side/piggyback RDS that better fulfill that ability. Additionally, I’ve used some FFPs with a very thick (relatively) crosshair. My old IOR was a whole .1mil, and on 18X was pretty coarse. It didn’t really matter for the kind of shooting it does (did) either.
It’s a mid/long range precision optic, not a benchrest/ELR optic. I don’t need an ultra-fine reticle to make hits to the level of range or precision required from an AR-15/10. If I were shooting benchrest/ELR, I don’t think I’d need the benefits of a FFP mil reticle either, with bench it’s not needed, and if it’s ELR I’d just use a maxed out SFP optic for holding mils, and a more precise method of ranging than mils. Now add in the added complexity and inherent added cost/room for mechanical error.
For me, it ended up being a nifty answer to a question that doesn’t need to be asked. Others will find more benefit in it than me, but apparently not too many because it never really caught on. YMMV.