The MM was not too popular with the mechanics, as I recall, since after every "fording" in salt water the wheel bearings had to be repacked.

One unique feature was the lack of a spare wheel. If the right front tire went flat, you simply continued to drive, since the weight distribution allowed it to be driven on three wheels. If any other tire went flat, you exchanged that wheel with the right front one. Despite this feature, a lot of the MM's I saw in RVN had spares lashed to the roof supports with comm wire.

I never had a flat, so I can't confirm this information.

Another feature of the MM was all wheel independent suspension, unlike the live axle of the jeep. It gave it a much smoother ride over rough terrain. My MM was laid up for maintenance once and I had to use a jeep. I was horrified at the difference. My head repeatedly hit the canvas roof driving over the same terrain I had negotiated smoothly in the MM.

The adoption of larger helicopters with increased lift capability removed the rationale for the MM's existence, and it was gradually phased out.

I had an opportunity to buy one once, at the Knob Creek machine gun shoot in Kentucky, but the very limited availability of spare parts convinced me not to risk it.

The Wikipedia article on the M422 discusses its features in detail.