[Mobhack]

Open topped vehicles have no armour on the roof, so people shooting down into them from above can kill crew (=hit points), falling artillery goes in like a stone into a bucket, doing the same thing. And they are not good things to be inside if assaulted by infantry that can simply flip a hand grenade into the tin bucket, or pop up on the deck and spray the inside with bullets.

Of course, some early amphibs were unarmoured - see the LVT 2 in the USMC OOB for example. Some later ones only had steel in parts. The unarmoured faces let bullets fly through just like a truck would. And being un or partly armoured was an acknowledged design defect of these early models.

Fire smoke to screen your approach, along with HE (produces dust/smoke) otherwise the things are only a little more safe than a rubber dinghy as used in river crossings..