I have a Roku 3 (4200x) and a 2016 Roku Ultra that I use with a 1080p (non-4k) TV, and for a few reasons related to your first bullet point am still using the Roku 3 as my main unit, and periodically pull out the Ultra to check for new firmware updates.
Reliability/performance: The Roku Ultra is much more responsive and outputs a noticably sharper image both on static menus and while watching stuff. For most people's purposes the Roku3 is probably "good enough;" it's considerably snappier than say an older Blu-Ray player or a cable box, and I wouldn't notice the difference in picture quality unless they were side by side. The one big thing that the Ultra shines on is its support for newer encoding formats; x265 loads on my Roku Ultra but I get an error about unsupported codecs using the Roku Media Player on my Roku 3. If you are only streaming online content and aren't playing your own content from a USB drive or NAS, I don't see a compelling reason to upgrade.
The one reason I don't use my Roku Ultra on a regular basis is that it is pretty glitchy. There are a few open threads about issues like white noise/static, HDCP errors, and the like that I've personally experienced, but that don't impact the 4200x. For instance, the screen on my Roku Ultra will periodically flash with funky static and the TV itself will emit a loud screeching noise until I switch inputs or turn off the TV. If these issues were resolved I'd have a lot more confidence recommending a newer Roku model or a Roku TV, but for now I'm sticking with my older unit.
End Of Life: Not particularly concerned about this as most of the services I use support a broad range of devices and appear likely to continue support on 1080p devices and older Roku units. I would prefer for the Ultra to be my main unit but until the glitches are ironed out, I'm sticking with the more dependable Roku 3.
TCL 55 inch 6 Series R613 (SN:YS00W7626058)
TCL 42 inch 5 Series (SN:YS00SA847038)
AVR: Samsung HW-N950 (5.1 DD, DTS+)