The idea is because they collapse in the mouth a horse cannot grab hold of the bit. Also good for leaners. I personally do not think it is a particulary strong mouthpiece allthough it depends what hands it is in. A snaffle could be stronger in bad hands than a pelham in good hands
I had a waterford baucher for my last mare, a heavy weight arab x cob. She was VERY heavy in the hand and could quite litterally pull you off over her head. With the waterford she couldn't lean or pull me out of the saddle, but it didn't stop her bolting, she could quite litterally run through it, grabbing her abundant mane was always the better option as she would then slow down. They are apparently kinder than snaffles as they do not have a nut cracker action nor point into the roof of the mouth. You might want to try using those rubber cheek ring things to start with if you are concerned you might saw the bit in you horses mouth. Also be aware that you horse may not like pressure on the tongue, in which case a waterford would be the last thing to use.
Neue schule are always helpful with this kind of thing.
I use a waterford occasionally for schooling sessions as Domi tends to get lazy sometimes and lean on my hands. I think it is a strong bit if you dont have sensitive hands. Every time he tries to lean on it, it drops away from him. I dont have to use it very often, I think of it as a schooling aid to be used occasionally rather than an every day bit. Normally he is ridden in a rubber straight bar snaffle.