I like the dynarette support. The guitar stays in place comfortably on my left leg and it is easy to place wherever you like at any given moment when you decide to play your instrument. I can also shift slightly while I'm playing if I need to adjust, so that, to me, is a great benefit. When I started playing (33 years ago... tempus fugit!), I used a footstool and I don't recall any alternatives back then. Of course, there was no internet and I only knew what I knew from teachers, books, and music stores. It was the footstool, folks, that's how you played classical guitar. These days, I no longer use a footstool, not because of any long term chronic issues but because I generally feel a lot of muscle tension in my back when using one. Sitting in that kind of asymmetry for long periods of time just can't be good for a human spine. I suppose if I had played more over the years, this would have likely developed into a chronic problem.
I've never heard of the ergoplay but now that you've informed me about it, I googled it. Looks interesting. You should look into Kris Barnett's guitar supports. I think he's working on version 2.0 which may not yet be available, but it is adjustable and uses strong magnets to keep it attached to the guitar. I think this might be a good solution because I suspect the magnets would do a better job of keeping it in place than the suction cups on the ergoplay, but that's just speculation.
2005 Kenneth Hill Signature double-top, spruce face, 7-fan
National Steel Dobro
2017 Gibson SG Traditional
2005 Gibson SG faded