I find a number problems occur with Smart TV apps that are not a problem with external products like Apple TV, Roku or Playstation. I would never recommend buying a TV based on these apps. I would focus more on image quality, durability, sound quality, form factor and cost.
These problems can include less buffering to handle internet capacity interruptions, lower quality user interfaces, fewer picture controls, lower quality sound and contract expirations. External devices like Apple TV generally have smoother playback and are also typically easier to operate. TV manufacturers have contracts with content suppliers like Netflix so they can have their apps in their products. However, once the initial contract expires the app will no longer work if they cannot come to an agreement removing a feature you wanted. Less control of the picture settings can reduce the image quality with a Smart TV app versus and external device like Apple TV. People using surround sound systems may also have difficulty getting more than stereo sound from these apps as well.