AIO's don't fail very often, and when they do it's usually the pump that fails from one of several possible reasons .. but sometimes the develop a leak
While AIO don't fail very often, they do fail way more often then air coolers.
If an air cooler fails the worst that can happen is computer gets hot and shuts itself down possible damaging whatever is being processed.
If a AIO fails and it's just pump failure than system just shuts down when it overheats, but if it's a leak it can (and often does) drip into electronics (like GPU from CPU above it) killing it when it shorts out.
Some will say damages are covered under warrantee. Problems are multiple from first even getting a reply from company, then trying to prove to them it was their products fault, than trying to collect. As often as not they won't answer, or want everything shipped to them (at owners expense) for them to analyse, then usually say it's not their fault and want you to pay to get your stuff back. Even if they do admit fault the usually only pay a fraction of cost of replacement By this time your system has been down for several months and you've likely already bought new bits and pieces.
But with air cooling about the only thing that can fail is fans, and they usually make plenty of noise for us to know we need to replace them before they actually die. Even if cooler only has one and it dies, we can use almost any fan until proper replacement is in hand.
Sure, occasionally TIM can pump out and/or dry out, but again we can see thing are getting warmer and warmer long before they get too hot.
Long story short.
AIOs cost much more than air coolers
AIOs have much higher chance of failing
AIOs have much higher chance of damaging components
AIOs can fail with little to no warning.
And they only cool slightly better than air when new, loosing the difference as pump wears.
So I'm going to continue using air coolers and don't even look at AIO.
If new builds get to the point of needed more cooling than air cooler do (and I don't expect that to happen), I'll do like I did 20+ years ago and use custom component loops. then we had t make our own waterblocks, use car radiators and aquarium pumps. Not we can buy everything off the shelf, just cost way more money than air cooling and takes way more time to build, install and maintain.