Dangerous is a strong word here. Yes, this feature does make browsing the web safer, but I would stop short of inverting that statement to mean that disabling it makes the web dangerous. It primarily protects you from sites engaging in social engineering of some kind: these can admittedly be extremely sophisticated, to the point of fooling most very technical people, but generally speaking it's still mostly avoidable with some care.

I would recommend most people having a safe browsing feature enabled, but I wouldn't fear-monger those disabling it either.

It's also worth mentioning that Mozilla provide their own service here -- Shavar -- so one needn't use Goog