I think it just depends on the company. Out of all my IEM's that I have ever used that used MMCX connectors the harmony had the worst out of all of them.
One thing I really liked about the A83 was that the cable was fixed to each earpiece and didn't swivel freely like most other earpieces, which I think adds to the long term durability of the connectors.
Same experience here, only problem I ever got with MMCX was the FXA7 demo unit I tried (disconnecting on one side at the slightest motion) and a few cheap aftermarket cables (30$) with cheap MMCX connectors getting loose over time (1-2 years).
Never got a single problem on the IEM side (the female MMCX connectors).
MMCX free rotation is usually a bad design choice, it means the contact is already loose at the beginning and will get looser with time. If you start with a tight connection (only rotates when forced to do so), the loosening over time will take MUCH longer before it gets so loose that you get disconnections.