Just my 2¢:
My wife has a smart TV for her monitor because, for some reason, she can see it better than all the monitors we tried out on her eyes after several optical surgeries. My daughter has a larger smart TV for a monitor, just because it gives her an actual TV in her bedroom.
In my limited experience, I hate having TVs as monitors. I like a device to have a single function....a monitor as a monitor. You turn-on the computer, and the monitor displays the video output of the computer...no fussing around with remotes to get the proper video input, and the monitor is optimized for the job it's supposed to do.
When the year rolls-around that I want to watch TV, I'll buy one.
As to how you know if a monitor/TV is actually 120 Hz refresh rate / FPS, that's when you dig into the technical specifications...not the brochure text, the actual technical specifications sheet; and sometimes you have to go a-searching to get that.