Many/most of the built-in interfaces of smart TVs are slow, buggy, and ad/tracker-ridden and have a limited upgrade path, if any.
Since there are effectively no dumb TVs on the market anymore, I bought a fancy Sony TV with AndroidTV built-in, hoping that'd it'd be decent enough to use out of the box. Nope. Slow, laggy, and some of the big name streaming apps from the app store just plain don't work.
Also when the TV updated itself it broke HDMI-CEC for a good couple months before another update fixed it. Screw that.
Bought an AppleTV and it's been great. Unplugged the SmartTV from the network and treat it as a dumb TV now.
Yes I know AppleTV is tracking me, too, but at least the user experience is great with no ads.