I lean towards SSDR #3 reasoning.
While I think Immunity to Normal Weapons is powerful, I don't think it is over powered. It just takes heavy weapons to put down large spirits. And MGL-12 properly employed or Machine Gun Fire properly employed can deal with most magical threats (there is however, collateral damage with the first, and heat with the second).
I think Spirits appear overpowered because players have spent time focusing on magic, but not on coordinated fire tactics, largely because most players aren't good at teamwork (or don't want to be). The reality in combat is that lone wolves that try to go it alone usually die. GMs rarely employ fire team tactics against the Spirits or the Troll Tank, but they should.
Don't get me wrong, I certainly think spirits are powerful (cheap, flexiable), and there should be balance. Reinforcing reasoning #3, I don't think physical adepts are a key part of that balance.
Observation: Elemental Missile + return of item attunement is a large shift to physical adept riggers being back as the apex predators. Magical (Elemental Missile) Heavy Machine Gun burst (8P) - Logic 6(

+ Engineering (Gunnery) 6(

Attuned Vehicle Initiate Grade 2 = 18 dice easy, after a few runs. A real attack helicopter crew is a match for several spirits.
It is major shift from previous additions, but not radically changing SR Missions play. Will increase players choosing adepts. I would guess by 1 per table (6-7 person team) at a Convention.
Clarity on elemental missile and grenade launchers visa gunnery should be made quickly.
Cheers,
Choppers