I personally always go with a dedicated device. The onboard software or hardware is often limited, and the track record of most companies to maintain their smart TV up to date and secure is rather abysmal. At least with the dedicated device it's rather easy to swap if something happens.
I have a LG TV and can speak specifically about WebOS. It is quite a nice OS. Very smooth. LG didn't make WebOS, they bought it. You can use the LG remote if you use built in apps. The built in apps are usually better than the ones you get in all but the top of the line shield or apple TV because they are made for LG TV and access the hardware directly. The built in apps will support both Dolby vision and HDR, something not true of all boxes. They will also handle 24fps correctly, while external boxes will usually just up convert them to 60fps. And the TV up converts hd to 4k better than anything that's not the shield or apple TV
You can hook a hard drive to the TV's USB port and play back just about anything with the built in media player, including 4k blu-ray rips. It will read NTFS, too.
The downside is the LG store is very small. It is missing a lot of apps. But if the apps you want are there, then you're good. It does have the major ones Netflix Amazon Disney YouTube YouTube TV. HBO (2018 and later models)
I use the built in apps whenever I can, and use external box to fill in the gaps.