Real Madrid is a big team, so it's natural to expect a coach who can deliver success quickly, but it won't be easy for Xabi Alonso.
I think Madrid is a unique case tbh. You need to win things asap because the fans and boards are not happy if you fail, whatever the reason is. I think that's why Ancelotti is loved. I think other big clubs have some leeway (1-2 seasons) for a new manager to fix things and rebuild the squad unless they're doing extremely stupid things like playing GK upfront or something similar. Well, hopefully things change with Alonso (although that seems unlikely).
In my observation, the uniqueness lies in Perez mindset, he has fired several great managers and some of them after winning trophies in the previous season. Perez ambition to win trophies every season to increase the club's income has made several great coaches fall victim, including Zidane, who has won the UCL trophy three times in a row.
Firing coaches has become a tradition at Real Madrid, and it will continue if the new manager fails to win a major trophy, including Alonso. This situation is very different from other big clubs, which consistently give coaches more than one season. Despite the fact that Ancelotti failed to deliver a trophy to Real Madrid in his final season, I don't think there was a compelling reason behind his dismissal.