There are a few things that can make a leverage hackamore harsh: the length of the shanks, the noseband, and the positioning on the nose. The longer the shanks the more the pressure of the reins is multiplied. A wider noseband is going to be gentler, while a thinner one will be harsher. There are a lot of different nosebands out there- leather, plastic-covered chain, stiff rope, etc. The lower the noseband is placed on the horse's face, the harsher it can be because it's sitting on a more sensitive part.
(ok, that second link isn't technically bitless, because it has a chain piece that goes through the mouth, in addition to the hard noseband and chain curb strap. I feel for any horse being jerked around with that one :( )
Mechanical hackamores are very similar to curb bits as far as their usage is concerned. 99% of the time, the severity of the hackamore depends solely on the use of the rider's hands.
HOWEVER, there are some hacks that are harsh no matter what you do, IMHO.
The chain nose JW hack that EHG posted is an excellent example.
Another is the "ladybug" hackamore
And also the "quickstop" hack
I don't like those simply because they are wholly made of metal and there is no give to them....and I've never seen even a single person ride in them with good enough hands to keep them from being terribly harsh.
In addition to what has been said, there are issues with certain bitless bridles that even once the pressure is released by the person, the bridle exerts pressure. That makes it extremely hard to train horses and results in confused horses.
I can't find it now, but I found a spiked hackmore when I was doing my project. I've heard of people nearby making their own noseband by using barb wire. I normally wouldn't trust that, since it's basically 4th hand, but there are some horses with suspicious scars circulating...