Smart TV's, particularly earlier ones, are at a disadvantage for a few reasons. One is hardware, most of the hardware is lacking, compared to Smart Phones, tablets and computers. Take Flash as an example, which is not renowned for being well optimised, a version of Flash intended for mobile devices was modified for Smart TV's and with each update of Flash needed to be updated too and to be optimised for Smart TV's and to accommodate for their various hardware configurations, the actual end result for those manufacturers who kept Flash on their TV's? It ended up sluggish and would sometimes run into memory problems and not suitable for streaming something like YouTube. So nowadays, Flash is not as widely supported on Smart TV's, especially as Adobe dropped Smart TV support for it. On top of the hardware limitations, there is no actual unified Smart TV OS and manufacturers have developed their own inhome OS's, which get improved over each generation. So there becomes the issue of supporting multiple manufacturer OS's. This means that when a TV becomes "out of date" so to speak, they may weight whether it is worth them continuing to support it or not. Annoyingly, it can be not. YouTube isn't the only one to drop it either. I expect with Smart TV technology this will become less and less of a problem.
So personally, I wouldn't rely on my purchase of a Smart TV for its apps, maybe its features, but not its apps. As with a smart phone too, apps have the potential to disappear (I am disgruntled that SE ceased the Android game they released with FFXV, it was a fun game in its own right). Although usually when app support is dropped on a Smart Phone or an Android version, it affects a tiny number of users as most people upgrade their phone approx. every 2 years, not so much their TV. Hence the "Smart" model on a TV isn't completely advantageous.
Fortunately, there's often ways around it, you can pick up a ChromeCast for only £30. I've got one, but then in my room I don't use a Smart TV (we have one downstairs) as I am using a monitor, but as my monitor has a HDMI port, I can just hook up my ChromeCast.
I guess sometimes when new technology does come out, there are things to iron out, advantages and disadvantages and problems to sort out. Hence I normally try to wait until I see how the technology progresses until I jump in and for when I do jump in, what I adopt. Hence I've been cautious about VR and have been watching how it develops. And with 4k, I am glad I did not jump into the deepend, because copy protection ended up screwing that one up, because the film studios decided everything has to work with HDCP 2.2, it is not anything to do with hardware being capable to handle 4k content, but to make it near impossible to record it. Which I am moderately disgruntled over, because I have a 1440p monitor on a PC capable of handling 4k just fine and 4k YouTube videos look amazing on it and was on the look out for a 4k Netflix or a 4k BluRay support I could upgrade on my PC. Now I am essentially penalised because my monitor pre-dates HDCP 2.2 and I'd have to not just get a 4k BluRay drive, but upgrade my CPU and/or GPU.
I think in that regard, they've gone overboard with copy protection, because for the first time the thought in my head propped up "I'd be at a much better advantage if I were a pirate, because all systems get broken eventually" but I am also a lawful person and do not agree with piracy and would not make myself a hypocrite.
