This unit has to be approached very carefully. It used to be the most overpowered unit in the game, being able to reinforce whilst having the quad upgrade and having insane suppression. Flame HT used to be this op too.
These units will never be balanced..
While that is most certainly true the reinforcement + upgade option was removed years ago which made the 251 is a choice between a cheap support vehicle and an effective glass cannon, while the M5 became a much less appealing choice. They both got the reinforcement ability removed after weapon upgrade which is more than resonable and actually made the play experience much more flexible by forcing players to make this choice. It was basically a textbook move on how to make an excellent balance patch.
The problem arose later, when they for random reason they decided in a later patch to remove moving suppression on the move for the M5 even at a time when hardly anyone was using it in the first place.
I'm not saying that they necessarily need to return this option, but as it is now the M5 is basically significantly worse and a lot more expensive than the 251 counterpart. And yes I know that you can never directly compare units in an assymetrical game, but the fact remains that one is popular and the other could pretty much be removed without anyone ever noticing.