There is no guarantee. Everything, and I mean Everything, is subject to Murphy's Law. If it's going to leak, it will. When is a different question.
I've only ever had to fix 1 pc (clients) that had a leak from an aio, and he installed it himself. He was in a hurry to get the pc back up and running, so just shoved the hoses in, putting undue torque stress on the pump fittings, causing a break in the seal and a consequential leak. My boss (at the time also fixed one). I've installed over 100 aios (at a conservative count) and never had one come back to me.
Leaks can happen. Pumps can fail. I've had a Phanteks aircooler with a warped base, Noctua aircooler with a leaking heat pipe. CoolerMaster/Corsair fans DOA. **it happens. You are at the mercy of the little guy who just finished the component you bought, on a Friday at 4:59 so rushed it or on a Monday morning after fighting with the wife Sunday night.
No guarantee. There's over a million aios manufactured and sold every year. And 10 videos on YouTube about leaks from this year. It's a pretty acceptable failure rate, even if there were 100 videos. There's an exponentially larger amount of psu failures, gpu failures even storage failures than there are leaks from aios.
Best warranty I know of from personal experience? Evga or Corsair. The pc my boss fixed was from a hose that blew on a Corsair aio (old style plastic hoses, no longer used) and sprayed everything inside. Corsair replaced the entire pc.
Theres only a handful of manufactures of pumps and rads used in aios, mostly Asetek or Coolit designs some with custom details like rgb/argb etc. So the basic performance possible is pretty equitable. The differences will be with individual fans. Plain fans work better than rgb/argb. Rgb/argb can look better. Or be more desirable. But overall, the differences are minor with the big name brands. Corsair, Thermaltake, Fractal Design, CoolerMaster, Evga, Nzxt, all similar. Just depends on the model, the included fans, the gimmicks.
Gonna withhold judgment on the new Arctic for a while. The previous gen units were not too shabby at all, got very decent performance at very good sound levels. The only issue was the coolant used, I'm guessing there wasn't a good mix of anti-fungal in it, so pumps were failing on some ppl at the 1-2 year mark after getting bunged up by bad coolant. Shame really.