XF70 = I doubt that is the case since this is a 2018 Sony entry level 4k TV . It may be possible at some point the app itself will stop running on a specific OS version but this can happen to all TVs eventually .  However the model you're talking about are of the 2018 TVs , it is unlikely they have gone unsupported within a year.

XF75 = More or less the same as XF70 but you are paying more for the Android OS , 5Ghz WIFI support.

LG UK6300 = LG 's cheapest 4K TV which utilize the RGBW panel .

UK6500 = LG 's mid range TV , its a bit better for watching HDR than UK6300 .

NU7100 : Samsung's most basic 4K TV in 2018 . Again , I don't think the claims are very credible because this is Samsung's 2018 model , so the " having issue after a few years " can't be true since we are still in 2018.

NU7400 : Like NU7100 , but with more features such as cable management , WIFI 5Ghz supports , blue tooth , etc etc . Also supports a Wider Color Gamut which means the HDR performance is a little bit better.
There are a lot of discussion and ranting about TV durability in general  . Most people only complain about their TV when it broke down , people who have never encounter any problems are usually quiet.
Therefore , the number of complaint about a specific TV brand is usually not a direct indication of the TV durability and manufacturing quality control.

For eg , a lot more people QQ about Samsung TV . That is only because Samsung sold the most TVs ( not in the premium market ) . Someone bought a Samsung TV today and it went haywire tomorrow, obviously he is going to make a scene in the forum .  Then we have the other guy who bought the same model and have lasted 7 years without problem but never say a word .

My verdict is simple , If I got a defected unit from one of these major brands , I will look at how efficient and how quickly they can hold up to their promise , to replace or to repair the TV . As long as they are willing to fulfill their promise , they are a good brand.  It is unreasonable to expect every TV unit to last 10 years because one guy said his TV did .
If you don't need the wide viewing angles , go for the Samsung . If you want to get a TV to watch with a group of friends and family , get the IPS from Sony or LG . If you prefer more apps selection , get the Sony Android OS TV but keep in mind not everyone like the counter intuitive and sluggish UI .
One is IPS and the other one is VA .
The Sharp is better for HDR and watching movies in a dim /  dark room.
The Sony is better for watching sports and fast action movies in a decently lit , large room.