1. If they are shooting army then they want to shoot from the fire points on the vehicles. If they are assualting then they have to wait a turn to assault (unless they has LR or open topped vehicles)
a. Note that a crew shaken or stunned result will not allow the unit inside the transport to shoot unless they get out which just makes them like any other foot unit or forces your opponent to waste a turn of shooting with that unit.2. A shooting army doesn't want to get out unless it has too or is doing some type of rhino rush. An assualty army will not get out until they can assault.
b. Note that an assaulting mech army outside of LR (not much we can do but then again you won't see more than 2 of them or the enemy will be outnumbered even to Nidzilla lol) and Open-Topped (hey venomcannon penn those vehicles HELL YEAH) will have to get close in order to guarantee an assault the next turn. This allows two things to happen:1. You can surround the tank not allowing them to disembark and therefore have to move again.
2. You can choose which unit they will have to charge (AKA gaunts lol)
a. Note that a unit that has to get out to shoot is usually a unit with weak armour (IG, eldar, ect) or Space marines (wow bolters and a melta gun or flamer "real scary"). Also note these die to BS or TL Devourers quite easily.3. If they do get out they are just like any other foot army except they move a little fast for a couple of turns. This takes away their advantage.
b. see the above but note the same thing to them dying.