From my experience with DirecTV Now, AppleTV4, and a 4K TV with Roku built in, the performance of DirecTV Now has been better on the ATV than on the Roku. From my understanding, Roku TVs are treated more like Smart TVs for development than as standalone Roku, which is why they are still working on fixing bugs in Roku app than my ATV. If I had a stand-alone Roku, I probably would not have these issues.
I recently purchased a TCL 6 series with Roku, and have a ATV4 connected to it. I also have a ATV4k that I got from the Directv Now deal, but I gave it to a family member because the 32GB was not large enough.
Due to my experience, my opinion on the matter have changed a little. And, this is based off of my ATV4, not the ATV4k.
I have been really impressed by the TV's built-in Roku apps. While there are a few things that I like about tvOS's version of apps, I think overall, the Roku version is much better. While most of the apps are mostly identical, there are little differences that I think the Roku's version has over the tvOS's. One of them being it is much easier to navigate with the d-pad of the TCL remote, than the touch pad of the Siri Remote. Another being that the Roku version adds little features like the side bar on Netflix. It still is not as good as the Netflix app on the ATV3 though.
One thing that is missing from the Roku, is quality games. tvOS is not known to be great for gaming, but it has much better selection of games, and many of them being free to try or play. The Roku's games are mostly cheesy bubblegum games that are usually free on tvOS, but they cost money on the Roku.
Obviously, the ability to bring the ATV with you while traveling is a still a big plus over the TCL TV, so this alone might make it worth it to purchase one.
About updates, only time will tell with this one. In my experience in the past, smart TVs don't get updates often and their manufacturers often end support for them after a relatively short period of time.
Overall, I am very impressed by the TCL. While I am not getting rid of my ATV4, my family uses it about 20% of the time that they used to. This might also be to the fact that I cannot get the ATV4 to look right on the TCL, the colors look too washed out.
I had plans on getting a 64GB ATV4k soon, but after using the Roku on the TCL, I might hold off a bit. Also, based on my first impressions of the TCL, if I replace the TVs in my other rooms, I probably will get ones with Roku built in, and I doubt I will get any new ATVs for them.