I’ve only taken my Fab Def mag out a few times so can’t comment on durability, but mine does feed and function just fine.
There’s not many posts on the Fab Def mags, but I’ve not seen anyone post a failure.
FYI AIM Surplus had surplus mags in good condition for $16.
And the AC Unity mags for VZ58s is a cheaper alternative to FD. Never used em in my VZ but got some for my AK and they’re thin plastic but metal reinforced front n back, no issues.