Concrete in general can fail from simply being mixed poorly, which the slump and strength tests used on each batch are meant to address. But also additives can sometimes be added to concrete, which in some cases react with surfaces/chemicals over time and cause all sorts of problems.

For reinforced concrete failures, see here: https://en.wikipedia.org/wiki/Reinforced_concrete#Common_fai... https://www.usbr.gov/ssle/damsafety/risk/BestPractices/Chapt... By the time there's spalling the reinforcement is already rusted and breaking up the concrete. Of course you can say "Oh but we know all that now, so just design it right and you're good!" Sure. Unless you make a mistake, in which case you won't know it until the structure is crumbling. And remember, "it's a monolith", so good luck with those repairs.

I'm not saying CLT is better - but let's not kid ourselves, reinforced concrete isn't perfect. It's just cheap, strong, and easy to install.