I was actually completely happy with the device until I made the horrible choice to install the update a couple months ago.
It's gradually getting slower and laggier, the wifi keeps disconnecting. Netflix has problems streaming shows in 4K (titles that usually have 4K available don't show that option on most days, only HD. Some days later it will show 4K again and then back to only HD again).
Also, the remote doesn't stay connected and the voice search is unbearably slow or just doesn't work at all. The sluggishness of the device is probably the factor that causes the most anger.
/rant
It's working great for me, none of those issues since the latest update.
You've done a full factory reset, right?
Also, how's your internet speed? Switching from HD to 4K and back and forth sounds like a speed issue maybe?