I had ghetto tubeless on my MTB. In my first race, my front tire pressure went low (the tubeless setup was a little old). I went through a turn really hard and the tire burped off the rim. I carried the bike the remaining 4 miles and 'finished' the race on foot (Then the race published their results with my name as a DNF. So what if I took an hour longer than everyone else!?!?).
I couldn't get the tire to reseal later, so I put a tube in. A couple months later, I was prepping to go on a ride and the rear tire had lost its seal. So I was late for the ride trying to find a tube.
I've seen tubeless specific tires and rims seat and seal without having any sealant in them. That's a bit more confidence inspiring. After my experience, I have no faith in ghetto tubeless and using standard tires in a tubeless setup.