Wrong.
I didn't say it wouldn't work at all. I knew it would work (even on Mojave and later). I actually bought one, specifically to drive a second monitor. Still got it, in its box now. But, in my case, it was only as boot screen/backup for a non-Mac EFI GPU. No Metal support. No 3D acceleration worthy of the name. Performance is much worse than a 5770, let alone newer cards. If it somehow does everything you want it to do as your only GPU option, fair enough- particularly the way GPU prices are at the moment (I could still get my money back for my RX 580 after four years of use!). GT120s are still cheap (£25 on eBay). But you could have spent a bit more, driven all those monitors from one card, at much higher performance, given yourself the option of running Mojave and later OSs, and perhaps be wondering why you didn't do it sooner.
As I said in my earlier post, fair enough. I can't remember HS being any problem to install on my 4,1, but it was some years back. Also it was on an SSD, which (even running on the internal drive bus, let alone on a SATA PCIe card) is way, way faster than HD. Brilliant bang-for-the-buck upgrade. You'd need a Metal GPU to install Mojave (installer won't work with any incompatible cards, like the 120s, fitted, even if no monitor attached to them).
