The author seems to have a decent amount of experience using many services. He does breakdown each by price, memory, and any cons about them which is helfpul.
99.9% uptime is 0.365 days (8 hrs 45 min) of downtime.
99.99% uptime is 0.0365 days (52 min 33 sec) of downtime.
It's also hard for the average person to catch unless they have users that are going to complain or a secondary service to constantly test the connection. And if they fail to deliver 99.99%, you just get a proportional credit. The benefits of advertising 99.99% usually outweigh the consequences of failing to deliver.
Edit: It also always depends on what uptime really means. I have one hoster who regularly does "network maintenance" where their entire network goes down for several hours. This can happen several times per year. So there's no easy failover.
It's so incredibly annoying. A few years back even their corporate site went down during such outages. They have that fixed now but I can't move all sites due to political reasons...