Well if it isn't illegal.....
But I'll be the first to say I wear a hi viz when I commute (during the day). To each their own but if there is some small chance that a motorist might see me because of it then I'm happy to take that chance. FWIW anecdotally I feel like there have been occasions where I think a motorist has noticed me as I cycle up to a junction because of the hi viz. Can't prove it mind you, but I have to think a moving bright day-glo orange blob has to help at times. It does seem at times there is a little bias towards it being "uncool", personally I'm at an age where I just don't care and wear what I think is best for me.
Interestingly when I buy a h%^met, I always want one that is a bright colour for the same reason -I remember some years ago it seemed like brightly coloured ones had completely gone out of fashion. Mind you, I do have a black one, but that's for off roading.
But to answer the OPs question: is it dangerous? I'm not sure it's dangerous by itself but I'd say it's safer to ride in more noticeable colours.