At a riding school I used to go to regularly (years ago) they used cavaletti jumps in the school.
I only ever had 1 problem with them (I was there for 3 years). Lack of impulsion and the horse I was on clipped the pole which pushed it upwards and brought him down. Being a deep sand school he was fine as was I.
A lady there did suffer some nasty bruising and cuts though as the horse ran out and she fell off and landed on the cross support. They got the plastic blocks soon after that.
So like I say, in 3 years of being around them, I personally only ever had 1 problem.
I wouldn't say they are dangerous, I was say they are just old fashioned and need a bit of respect compared to the plastic blocks. Either way, I have seen horses fall after they have hit a pole on a plastic block and it has shot forward and the horse has got their legs tangled in it.
I was wondering about getting my handy husband to make me some of these, but do worry about them getting caught up in the horses legs, but I suppose that happens with ordinary poles too. There are obviously a few risks with them, but non so more than say cross country jumps, or jumping walls/hedges etc out hunting.