What the manufacturer specs don't tell you...
I won't say that the manufacturers lie, but lets just say that they can be economical with the truth.
The TV manufacturing industry is incredibly cutthroat. Samsung and LG both make a profit by virtue of their size and market dominance. Sony and Panasonic both posted horrific losses for their TV manufacturing divisions. Other brands eke out a living either by dominating a market sector (budget TVs from Vestel and UMC etc), or selling the same product under different brands (Vestel and UMC again), or by specialising in certain markets (Cello) or they have some investors with deep pockets.
Whatever is going on, it can't have escaped your notice that for any given TV size in the UHD market that there's a really wide gap between the cheapest and most expensive products on offer. The difference is way more than can be justified simply by saying product 'A' is a premium brand.
In the budget end of the market, once all the fat is cut away, then the only place left to reduce costs is in the component quality. A big part of this is the panels. Older generation panels are cheaper but off the pace technically. They find their way in to cheaper sets. The same is true of the motion processing and image upscaling tech. Budget sets just aren't as good, and with some of the poorer ones you're going to see jaggie lines and colour-by-number image processing artefacts if you get within a couple of metres of a 40-43" screen. The TV control software is similarly compromised. The range of picture tweaks I can make to really dial in the image on my mid-range Panasonic GX800 dwarfs what's available in say a Sharp or a Vestel-made Panasonic.
Another significant image quality difference with cheaper sets is the brightness of the backlights. Cheaper tellies just aren't as bright. Unlike LG though, they don't generally try to overdrive the lights that are there.
Other differences include build quality, the evenness of the backlighting, the range and speed and responsiveness of any apps, the sound quality of the speakers. This list isn't exhaustive.
Hunting the unicorn
"
What about getting a good non-smart TV?" or "
Recommend me a good non-smart TV" are comments that come up fairly frequently. They're both based on the idea that TV manufacturers are still producing good quality non-smart TVs. Honestly, you'll have more luck finding a unicorn. That's a mythical creature too, just as this 'great but not smart TV' is too. Neither exist.
The non-smart TVs that are around are very much the bottom of the barrel in terms of performance. Smart features are now ubiquitous and there's no price premium for them. In short, don't buy a non-smart TV thinking that it's cheaper
-
However, the idea of using an external smart device is a good one
The cheaper the TV then the less processing power it carries to run the apps. This means that the apps themselves taps in to streams from iPlayer and the rest that are written in a way that they'll run on low-spec hardware. This is part of the testing procedure that certifies the TV. The implication of this is that you might be accessing iPlayer on your budget TV, and so is your neighbour on their high-end OLED, but you're accessing different streams even though the content looks the same.
From time to time old and redundant streams are discontinued. I don't know what the trigger is for this. It's certainly not a best guess that the particular hardware is no longer being used by consumers. There have been several of these extinction events that left smart device owners with inoperable apps.
The point here is that a smart device with more horsepower is more likely to be supported for longer. Smart-TVs, -BD players, -PVRs, and smart surround systems are going to be earlier casualties than Roku boxes, Firesticks and Apple TVs. If you want good long term support, and something that will run apps smoothly and is likely to support new services when they arrive (e.g. Disney+), then get a Firestick or similar. For £40-£50 it'll run the same range of apps as a TV (no subscriptions required), and if it does eventually conk out then all you're replacing is a £40 HDMI stick and not an entire TV.