I have a Roku 4 and have never had any real issues with it. It is set to go to low-power sleep after awhile, and to 'run cooler'. I've had it for awhile and it's been stable. It is connected via Ethernet. It's 4400X, software 8.0.0 built 4128-17.
Yesterday, when I woke it up after it had not been used for a few days, it did not display the little ad to the right of the channel list. After a few seconds, it rebooted. It was fine after that as I watched some Amazon and Netflix. I then was looking over a few new channels, and added [adult swim] and Cartoon Network with no issues.
I then decided to try and re-add Travel Channel. I used to have it, but as a Comcast/Xfinity customer, it needed a workaround for authorization, and that had disappeared a few months ago. I went to the Travel Channel site to enter the code, and before entering the code, I got the 'authorized' message from Comcast on the phone, which I was using for the web side of the setup. Quickly after that, the Roku rebooted again. After the reboot, I went back to the channel and tried to add again. This time, I was able to enter the code and it passed, but again, the Roku rebooted instead of going back to the channel's main screen. After the reboot, I went back to the channel and it was authorized (shows 'xfinity' on the top.)
I'm not sure if there was some minor update recently or not that caused some of these issues. I have a Roku Express in the bedroom, and it has 8.0.1.