Well, all I have to go on is experience, sooo.....
I couldn't do that many pull ups without help,
I used the assisted pull-up machine for about a month, twice a week.
My workout told me to do 50 total, I would do 25 open and 25 close grip
At first I couldn't do 25, I had to take it in 10 or 5 at a time. After a month I was doing 25 at a time
and lowering the counterweight to 70 lbs.
I decided to try them without help and surprisingly did all 25 in a row by my self. I took a short break and then did the other 25.
So from personal experience, I may not ever use it again, but it definitely did its job and now I can do pull-ups on my own and my spine hasn't ever had any problem and I never felt like it was out of it's normal position...
Is it dislocated when you do pull-ups naturally?
If not, I would see no harm in using the machine which would be practically identical to someone holding your legs.