My MPM tailgut is working out fine after several years now. The carrick knot doesn't give up as of yet and I managed to get some semblance of afterlength tuning after dicking around with it years ago.
I have the MPM tailgut as well, and I'm fairly certain it is not nylon. A while ago, someone find the product that it really is, and I recall it being rated for well over 1000 lbs of tension. I'm very confident in mine, but would never want to use a nylon one.
-Edit- I just found the product that is used for the MPM tailguts. It is the 7/64" Amsteel cable.
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It is rated for 1,400 lbs. Definitely not nylon chord.