Movement against vehicles is tough, Movement on people still doesn't beat the speed of a car due to movement rates nerfed, you can't run through town or over highways, you're less squishy in a car, you draw less attention to yourself in a car.
Spirits cannot sneak, so they make sounds even when Concealed (aka Improved Invisibility). They are clearly visible in the Astral, whereas Drones are not. No sneaking means no decent pool to hide from astral perception, whereas a Covert Ops character can try. A spirit observing a building draws attention from security spirits. A drone covertly observing a building, is far less noticable. A metahuman being subtle, idem.
Magic-detection security exists, so good luck sneaking a spirit into a building that's GloMoss-wired. At least the Covert Ops can try to bypass mundane security.
Spirits are NPCs, and with limited knowledge of this world. They may be very intelligent, but not when it comes to many areas that count. Speaking of: Spirits cannot interact with technology. You want a matrix screen read, or buttons pushed? Yeah, good luck doing that through magic means.
I'm not saying there's no balance problems, especially not when it comes to their combat Powers, but Spirits are no replacement of actual PCs. They're just a tool. And some of their problematic powers in SR5 are well-adjusted in SR6, which already helps. Plus the new Summoning rules means no more 'I call upon my 8 Bound Spirits and 1 Summoned Spirit to butcher all my enemies'. So yes, there's room for improvement, but I do not believe the 'Spirits break the entire game and make entire archetypes obsolete'
My only problem with Immunity is simply that it's imbalanced due to not being adjusted to the new damage codes. A Force 6 Spirit can soak damage better than a Roadmaster. Spirits should not ACTUALLY be immune, but they need to be more solid. F/2 autohits suffices at that.
If we compare: SR5, vs AP -5, Force 6 Spirit has say 5 Body, 12-5 = 7 Hardened Armor = 12 soak dice + 4 autohits = 8 soak. So an Ares Predator with APDS would on average soak rolls basically apply its net-hits as damage. Regular rounds would face 11.7 soak, so at 4 net hits you would do only 0.5 damage or so on average.
In SR6, Force 6 Spirit has say 5 Body, 6 autohits = 7.7 soak. The Ares Predator is down to 3P, let's add Explosive rounds. So that means at 4 net hits, you do like 0.5 damage average or something. (Not going to math that out now.) In other words, the specialised ammo in SR6 is comparable to using weak ammo in SR5. The specialised ammo in SR5 actually made a difference and a fair fight.
If we're looking at the higher-end weapons, they used to be up to 15P, so you actually could gain significant damage on that same spirit. Now, the increase is just +2. That means specialised ammo in SR6 on a tough weapon, has the same result as a Predator with APDS in SR5.
That's not scaled well. If, however, Hardened Armor is properly scaled down to Rating/2 autohits, you still do like 0.5 damage less than in SR5, but at least it's only 0.5, not 3.5. That's far more balanced.