Recent history shows us very clearly that TV manufacturers are rubbish at making and supporting the smart features of their TVs.
Two things happen when the set is new. First, they only fit the bare minimum of processing power to support the apps out of the box. Second, they use hobbled versions of the apps to save a bit of processing power. Let's skip over the bloat ware that they add to generate a bit of revenue. The result is often a clunky and laggy user experience.
Over time the apps get outdated and become harder or impossible to use. Since they're custom-written lite versions, you don't get the same support and updates that the full fat version receive.
The bottom line here is that you can add a 1080p Firestick to any telly for £30. A 4K one is £50. They're both cheaper in the sales. These are infinitely better than any built-in apps. They work smoother and will be better supported.
Adding a Firestick to an existing TV is a great way to make it smart too.