Newer, more powerful Rokus (like Roku 3) can handle 1080 @ 60 fps video, especially when hardwired despite Roku’s overall recommendation of only sending 3.5 mbsp. This is why some folks say, “Hey works great… no problem!”
If you’ve got a so-so router or network, or an older/less powerful Roku, or you’re running everything via WiFi, that’s where things get dicey.
Everyone’s network is different and there is a lot of fragmentation in terms of firmware & hardware on Roku so it’s difficult to define what the ‘magic’ formula will be.