Isn't that basically the same as Slime, which is (from what I understand) widely reviled in the biking community?

Something like 10k miles later I've had zero incidents where I had to fix something on the road. You do have to more maintenance (e.g. make sure you have adequate sealant in the tires periodically) but for regular year round use especially, it is a worthwhile trade.

On my tubed bike, other than 1 or 2 flats in the last 5 years, these tires have required 0 maintenance. Even after dusting the bike off from the winter, they pump right up and are ready to go.

My tubeless tires are a constant source of headache. They are constantly going flat, sometimes weekly, requiring more sealant, stuck valve cores, etc.

Overall, I'd say my tubeless tires require monthly maintenance, while my tubed tires have basically never required any thought at all. What am I missing here?

If you're only getting 2 flats in five years though yeah stick with tubes. I get 3-4 a year and they're much less of an event with tubeless, so the extra routine work pays off.

That being said, tubeless tires do require being used fairly regularly, which includes pumping them up (just topping them off) almost every time you go out. Right now I haven’t gone mountain biking in quite a while so I guarantee that my tires are flat

Different tyres have different wall constructions, and so some can leak a lot of air. I find that after I put on a new tyre, they leak air for the first couple days, but it's greatly benefited by going for a few rides (the extra pressure gets the sealant into all the nooks and crannies of the tyre casing, filling the gaps), and then maybe lose a PSI or two per week (when I run at 50psi).

I change sealant about every 6 months, just because it dries up. Not particularly due to fluid leakage.