I can't answer the questions, but I have two Samsung TVs one about 8-10 years old (1080p 55") and one maybe 10 - 15 years old (720p 40") that are both still going strong. The 55" needed repair once early on, I can't remember why, but it was warranty.

the panels are usually OK for the qled, but onboard android tv is underpowered and known for been buggy, i wont use it.

I've got a 43" TCL 735 that I use as a monitor, I find it fine for this use.  Compared to my normal Sony TV I find the colours a little washed out, and the Google TV interface is much slower.  But considering what I paid for it I am happy with it.

got 55" TCL that been used daily for the past 5 years

Absolutely love it. 4k picture is great, blacks could be better, but hey I paid $600ish for it and I added back led light which just brings the color on TV.

AndroidTV is OK, I dont like the latest update with suggested content, but can live with it. 

I had a play with one as I was considering it as a cheapie for the garage. The android on it was IME unusably laggy, and casting to it was very hit and miss as to what would work.

In the end a nicer screen popped up on trademe for cheap that will be great in my gaming room when its done (NEC Multisync monitor) so I got that rather than the TCL, but it was very close to getting the TCL and would just put a firetv or chromecast with google tv on it for youtubing if it was too bad.

Picture quality was fine once all the settings were fixed from the energy saving defaults. Didnt see any flicker to the image like some screens have, but there was some backlight dimming still going on.

I have had a 55 for a couple of years. As others have said, great picture, but the software is rubbish. We just use a Google Chrome cast. The sound is also ok - far better than Veon...

Had one a few years ago. Very average everything, some things were way below average, not one thing to shout about. worst thing was picture had a purple cast.

TCL tv are  atrocious, service is terrible and service people hard to understand and first time emailed twice in 3 weeks then called long time on phone waiting just gave me software updates 2 times(first time not fix) as tv was taking 2 to 3 minutes to start up and other problems.TV started playing up after about 2 and half years and only 3 years warranty when I got 2nd update only 2 days left warranty and software update seemed to fix it, then 3 months later no picture and backlight gone, rung to explain took ages with their broken English and they just told me warranty expired nothing they can do, asked where service agent in NZ they said there was none. They were just lying and didn't want anything to do with it as out of warranty and know there are problems with there tv as you have a look on trade me heaps of refurbished TCL tv being sold with backlight problems and they say they are official service agents for TCL .

In the end I got tv serviceman to have a look and he said would be $300-$400 approx. to replace backlights so only solution like fixing at cost of new tv or he could put in recycling at a charge or I could do. Then he offered me $150 to keep and use for parts which was good deal for me as was only going throw in rubbish.
This whirlpool site on TCL tv heaps of complaints and also mine twice on Geekzone.

https://forums.whirlpool.net.au/thread/3mn5j4l9?p=107

https://www.geekzone.co.nz/forums.asp?forumid=34&topicid=288605

https://www.geekzone.co.nz/forums.asp?forumid=34&topicid=281135

Avoid TCL tv's I know cheap but no service and very buggy software.

Its up to the retailer to sort it out and honor their CGA obligations so somewhere like JB or similar are on the hook if it breaks that soon.

I have a 3 year old 75".  its utter rubbish.

Colours have started to go on it, reds are a washed out orange for about 30s until they settle, it takes forever to turn on (android always hard reboots, ignores the settings for sleep) around 60seconds.  

I'll never buy another TCL tv again

The TCL sweet spot is in their smaller models, 40-55"

Not great out of the box but if you can be bothered fine tuning they can produce excellent pictures.

Smart features are meh and the large models are poor value for money.