Its a bot more expensive for sure but also more encompassing. You can reinforce and arm your infantry from it and as vip said call in arty which can be very useful. It's kind of a number of things smooshed into one and priced accordingly. Regardless, mobile aoe heals for dirt cheap is somewhat stupid frankly.
Forward assembly actually has a bigger hitbox that vipper conveniently ignored
The thing costs an arm and a leg to upgrade and gets blasted down pretty fast.
This is also why the arty ability you listed is useless, the short range and fragile nature of the FA makes it unusable because even a rak shooting at it will be out of range.
Not saying FA is bad, it's better than med bunker in team games. but as Brits I would swap it with med bunker in 1vs1 in a heartbeat when every 100mp here or there counts. Especially with the 450ish mp you need for Tommies upgrades.
Which goes in with why vipper was wrong and my original reply, Med bunker has the edge in 1s and FA has the edge in team games. Apples and oranges