I have paid my Fidelity Visa on and off with MO's for years. But I consider this my safest card, since I have a very large balance with Fidelity.
Currently I am sending one envelope of MO's in the high 4 figures to each of several credit cards. For one of these the MO's represent about half my monthly payments, for the others it is 15-30%.
So far so good. But as we know in MS, everything is fine until it isn't.