overall size and the wasted space. For huge vehicle to carry only six dismounts -- five if you leave one to pass 25mm ammo up to the turret as is the norm is borderline criminal IMO. That goes to only three or four if you're short a man or two in the squad (which is typical). The height of the vehicle makes it a shot or missile magnet. It is over-armed for its role; the TOWs encourage tactical misuse. It's range limited...

It's supposed to be an infantry carrier -- it's not, it's a light tank. Too light...

The vehicle was a compromise in too many respects. Instead of the needed heavily armored, accompany the M1 vehicle (like a Namer) AND a battle taxi for volumes of Mech infantry (M-113 updates) AND a decent Cavalry Scout vehicle (M-113 would also work for that...) we got a compromise vehicle on a drug deal between the Chief of Infantry and the Chief of Armor. The former would support buy of the M1; the latter would support buying the M2 and its M3 variant. Both agreed to give up something, Armor the Future Scout Cavalry System and Infantry the XM-8 Protected Gun system. Bad deal all 'round...

It's perhaps noteworthy that the two Cavalry Regiments in Europe at the time of adoption called the M3 Cavalry BFV the 'burning fighting vehicle' contending there'd be a trail of hulks all over Europe if the USSR were to attack. They also sensibly lobbied to get rid of that humungous turret and replace it with a .50 cal overhead weapons station (thus allowing 7-8 dismounts...) to lower the profile.