Nerfing guard towers

Range of obelisk, Adv. Guard Tower and SAM site reduced.
Damage of Guard Tower reduced vs. infantry and light vehicles; spread increased to compensate a bit.
Damage of Advanced Guard Tower nerfed, especially against light vehicles. Damage vs. infantry increased (though it's still low DPS). Spread also increased to compensate for its lack of power vs. infantry.
Turret damage mostly unchanged, but does less damage per shot. Damage vs. light vehicles increased to match damage vs. heavy armor. Damage vs. infantry slightly reduced.
Improve Hum-vee/Buggy machine gun vs. infantry

Hum-vee/buggy machinegun buffed to make them more useful vs. infantry. Currently they are rather weak. 
Damage vs. other types of armor reduced to compensate.
Flame Tank damage vs. light increased
BikeRockets buff vs. wood. Nerf vs. infantry.
Nod Artillery buffed. Esp. vs. Heavy armor

If you want to consider increasing the price as a result of this, that might be OK.
But as far as the unit's tactical effectiveness is concerned, I think these changes are necessary, vs. buildings and heavy armor in particular.

In order to keep Arty from being OP, its gun remains inaccurate. It's also slow and vulnerable to exploding.
Mobile SAM: Damage buffed. Range reduced -1.

Given that SAM sites have range of 8, that it would be appropriate if mobile SAMs also had a range reduction. Choosing 9 for now, since battlefield conditions are a bit different than for static defenses, and because turrets sometimes behave funny with respect to range and firing.
Flamethrower buff vs. vehicles

Flamethrower should be good vs. light  vehicles. Buffing them vs. light, and changing damage to kill most vehicles with 3 hits instead of 4. 
They are also getting an increase vs. infantry from 90% to 100%.
Their speed will be getting reduced to normal infantry, so I dont think we have to worry about them being OP. 
Slight nerf to damage vs. heavy to better define the unit as not a mid-game anti-armor unit.
Nerf APC gun. 

Reducing damage vs. light vehicles & buildings, since APCs are kind of hard to kill early game.
Increasing damage vs. heavy slightly so they are non-useless.
MLRS buff vs. buildings, infantry

Artillery is weak vs. buildings in CNC, so this needs a slight buff (ARTY also buffed). 
MLRS needs a slight boost vs. infantry, since GDI relies on it to. 
Spread increased from 15 to 16.
Nerf mammoth missiles

Mammoth missiles are a bit crazy, especially vs. buildings. Nerfing them a bit.
To compensate, increasing their spread a bit.

To increase missile efficiency (avoid extra missiles being wasted on overkill), their damage has been reduced from 75 to 45. They can still kill orcas in 2 shots.
To further avoid wasteful firing of missiles, the burst delay was increased, and the speed of the missiles was increased.
Buff Orca dmg. vs. Heavy. Nerf vs. Air

Orca damage vs. heavy currently is pretty abysmal. Needs buff to define unit as anti-vehicle (instead of being equal to Apache vs. vehicles/armor).

Orca nerfed a bit vs. air to put it on even footing with Apache in air-to-air combat.

ROF slightly increased.
Increase Chinook sight range 8 -> 10
Reduce Orca reload speed, from 24s -> 12s

Orca's reloading is so slow it can barely do anything. It's painful. 

This will make it non-useless in smaller groups. 
I am not concerned that this will make Orca OP, since air defense power has generally been buffed in the last year considerably.
A10 is no longer a targetable unit.

Units on the ground were pursuing A10s until they left the map. Seems inappropriate.
Reduce Harvester cost from $1200 to $1000

This is to make opening with a factory instead of more refineries a bit more feasible. 

Given that there is a maximum limit of how many harvesters can operate from a refinery in TD, lowering the price shouldn't be a problem, since there can't be harvester spam. How many refineries you have will limit how many harvesters you can effectively use. 
By comparison, RA harvester is $1100.
Flame Tank armor now Heavy. Cost reduced to $600.

HP reduced to 300 to compensate for new Heavy armor.
Flame tank does good damage but often dies before it can be cost-effective.
Since it is supposed to hunt infantry and light vehicles, this will help it do its job better. It will also be more resistant to flame infantry, which seems fitting. ;)
Mammoth Tank HP reduced from 900 -> 800.

Mammoth was a bit OP and kind of impossible to kill. This keeps it tough but makes it possible to defeat.
Minor: Adjusting ROT speeds

Capping light vehicle rotation speeds at 10.
Light Tank turret rotation will match vehicle rotation.
Stealth Tank rotation increased to keep it maneuverable vs. tough enemies like Mammoth.
Minor: adjusting sight ranges

Artillery, MLRS and Mobile SAM all have visual range capped at 8. This makes scouts more useful to fire beyond visual range.
Buggy given visual range of 8 to match Humvee and Bike.
Buffing Light/Med. tank HP

Tanks seem to die quickly in this game.
Giving them a buff in HP to increase their usefulness in the role of "tank".