It is the associated risk and I think it is best that it stays. Having both healing/reinforcing is a huge benefit right on the frontlines and if done correctly (even without a Major) you place so much pressure on your opponent that they can't help but forefit map control.
If you misplace your medic you should be punished for it.
You realize a 222 can kill it WAY faster then an M20 or Greyhound can kill an Ost halftrack let alone the OKW medic truck.
Benefits:
It comes early
It can heal
Has a Medic inside that can be removed
Sometimes it can even reinforce! See below.
Low Fuel cost.
So lets go over its drawbacks:
Cannot Garrison multiple squads
Slower Acceleration then a King Tiger
Slower top speed than all other HTs
Cannot Reinforce in non captured territory
Cannot Reinforce if territory is cut of from HQ
Low Armor and HPs the only way to reliably escape a SturmPio is to literally retreat the Ambulance BEFORE it sees the Sturmpio
But yes it comes early is cheap on fuel (but the Manpower cost is still high enough I dont want to be constantly replacing this thing)
I think the only way to make this thing easier to kill would be to make it immobile. I mean a single squad can easily kill it from full health before it can escape in most cases and that is just wrong. This is why most people leave it at HQ.
Manpower cost is enough and the Micro demand is so high its not worth the Squeeze unless of course you are the StephenJF, Cataclaw, VonIvans of the world. If I am fighting a battle and dodging potentially squad whiping Rnades all damn day I generally have a hard time sparing tasks for the Ambulance thats going to surely die in a few hits.