I used to use the plastic decades ago in the 1960`s before I owned Leica.
Last time I purchased some from B&H, I think a pack of 4 or 6, I loaded a few and tried them out. They would stop the advance of film at around 10/12 frames no matter how much was loaded. I had the same problem with Leica screw mount, M and R cameras.
I put them in the trash, cassettes naturally.
I bought a box of 60 Kalt ones from Calumet. Don`t buy the individual ones from dealers who have an open box as they get filthy before getting sold. Dirt is the main enemy of reloads. There are others such as the pressure the felt seals put on film believe it or not.
I have switched to Leica brass cassettes and a lot of problems went away.
Early M6 cameras were the last ones to support the cassettes, but you could replace the base plate of later M6 with early base plates and make them work. They will not physically fit a M7 or Mp body shell.
IXMOO for M cameras are hard to find. Filcas for screw cameras as easier, but do not fit M`s. IXMOO`s fit screw cameras.
Bottom line, don`t buy plastic for Leicas in any quantity until you know they will work.
The Kalts can leak a little oround the end covers but they have never damaged film.