I am currently using Nvidia Shield TV Pro and it's working well, but it does not support 60FPS on YouTube, I actually dunno if TVs in general support 60fps or can view it, but I would love to have an Android TV box which has the 4k 60fps option.
For reference my TV is a Philips OLED 4k from summer 2018 and unfortunately it's not working well with its apps which is why I am using a shield tv pro, I just miss 60fps, whenever I see videos of YouTubers saying that the vid is in 60 FPS or I see it with 60fps option on my browser on PC it makes me miss having that option on my Shield TV Pro. I hope at some point they will release a new box which supports it if no better box is out yet.
The Shield does support 4k 60fps make sure EDID is set to 2.0 for the HDMI port the Shield is connected to and the Shield should match to the TV what ever the highest resolution and framerate is.