Unfortunately the age of CERN means a lot of the technology is what could charitably be classed as "legacy". You could end up working on software that should've went away with the Aztecs.
As a software engineer I've often found myself in a development silo, which is to say I've been given a project and expected to go away and just do it; without much in the way of feedback. If you like working by yourself, that should be fine, but if you want experience of team-based development then you may not find it here.
I describe CERN as an old institution in terms of age, but unfortunately it can be rather regressive as well. I've witnessed colleagues being blatantly sexist and transphobic. I would hope that an institution on the cutting edge of physics research would be more progressive.