Nothing wrong with making a call at half time based on safety concerns.
Personally, when making a pre-match decision on whether or not the match should actually start, I always try and envisage how things are likely to turn out after say, 20 mins in. If I think there's a chance the match won't finish, I'll err on the side of caution and call it before KO time. I might pi55 both teams off with that but (IMO) it's better than ending it all after 1 team is 2 or 3 nil up.
Having said that, I've only ever vetoed 2 matches before KO in the 11 seasons I've been refereeing so generally, if I start, we finish after 90 mins.
Everyone is different though and I guess it'd be worse to start the match, have players moaning about the conditions before and during the game and then think to yourself "I probably shouldn't have started this fixture".