It is a complicated issue as it is with every large update. I don't know if they some problems like Bitcoin and Bitcoin Cash back then? But I would not know why that could be the case.
Whatever but these updates are probably discussed in the slightest detail and I am sure there is no consensus all the time between them. Especially if you consider that the development individually has money at stake, which is good for the community but also leads to discussions because all of those brilliant minds on the Ethereum development team has their own opinion.
But they can do so much testing that t should be relatively safe. There have been other forks although this is of course a huge one where all players have to agree. It should go well and hopefully have good effects.
You would expect that, but everyone knows what it's like when it comes to testing: it is a different situation under different conditions. The range of unexpected externalities can't be simulated. It's like in a penalty shootout in a soccer game where you hit ten out of ten in training but fail in the middle of a stressful game situation.
I am sure they will roll it out successfully but at the same time I understand how thoroughly they proceed.
It is also not like they are rolling out their first update. The fact alone that they have built Ethereum from scratch provides them with so much expertise that I don't see any issues arising they couldn't or didn't anticipate. Still though there is always some risk involved. Buterin himself says that repeatedly.