We have a couple remote ATM's that we run with Z3C's. The experience has generally been positive, but every once in awhile (every couple of months, maybe?) a handful of Z3C's or MX67C's will go offline. The devices will not come back online until someone goes on-site power-cycles the things.
We do not have the same behavior with Z3's or MX67's using an MG21 for their only uplink. If I were to recommend anything, it would be to avoid using the built-in cellular as the 'primary' connection on these two platforms.