The Ultra started as a poor device for Plex. But after several false starts and even steps backward it has finally become one of the most stable.
Also, for me, it is by far the best Plex interface. It is also pretty good for Emby.
If you plan to stick with Plex and you do not plan on the advanced video and audio features (I cannot attest to the Ultra's performance with the advanced setups as I do not use them) the Ultra is one of the best, most stable, clients there is, at this point.
One further disclaimer, if you have any plans to move to Emby the Ultra is a good Emby client but both the Fire TV and even more the Shield TV are better Emby clients but they are poor, at best, for Plex.
Emby's developers on Android based clients are VERY good but Plex's are less so.
On the Roku side it is Plex that is leading but not by the margin the Emby is leading on Android.
However anyone considering new hardware needs to be concerned that the new improved Plex interface seen on Android may ooze its way onto the Rokus. If that happens no matter how good the Plex/Roku programmer is the direction will be the start of a downward spiral.
But back to the Ultra. Roku just announced new firmware "8.0" and it is hoped, with some reason, that the new firmware will stabilize the product and make it as great as it could have been from the start. If I needed to buy a new client for Plex there is a very good chance it would be a Roku Ultra particularly since there seems to be nothing better in the set to box category for Plex with the exception of devices like the Raspberry PI running an embedded client. But those are single function and the Rokus have many other uses.