My problem with Roku upscaling is that, for video sources below 1080p, the quality is noticeably substandard. Some commenters here say it's blurry, which is true, but there are other processing deficiencies that I've noticed. For example, when comparing the exact same scenes in the same video sources with both a Roku Ultra and an Oppo 103D (using "source direct" video passthrough, which lets the TV do the upscaling), I can see that colors are slightly "off" with the Roku. The scenes in Star Wars: The Last Jedi that take place in a red room (around the 1h37m mark) are a good point of comparison: At 720p via the Oppo, the background is deep/intense red; through the Roku Ultra by comparison, the background has more of a dark orange hue.
I wish Roku would allow video passthrough because, clearly, most 4K TVs can do a much better job of upconversion. I can use my Oppo to watch lower-res video, but I prefer Roku's simple remote with a dedicated skip-back button. My Roku's wifi connection is much faster as well (802.11ac 5GHz vs the Oppo's 802.11n 2.4GHz).
Still, unless Roku comes up with some future software upgrades to improve picture quality, the Roku Ultra will be my first and last Roku device.
I feel most people don't even notice the difference because their focus is on the content, not the presentation. I'm from the other side. Call it obsession, but I want the best presentation possible. And that means letting me decide who will do the upscaling.