main disadvantage is the the price............you can get 2 6A13tt complete in front cuts for the price of one 4G63T on the ground and you still need rear suspension parts from a VR4.
So which engine is better, while it is true that the 4G63T has a lot more aftermarket parts letting you push to 500hp or more fairly easily, nothing like that exists for the 6A13. For most folks 280-350hp range is enough for some fun driving so I don't see a reason to even consider the 4G63T. A full exhaust including downpipes and increased boost will see you to over 320hp very easily, add an intercooler and a fuel controller and you'll be at 340-350hp.
The V6 is an amazingly smooth engine with no harsh explosive power and vibrations at any point in the rev range and it loves being anywhere between 1500rpm and 7000rpm with virtually no turbo lag at all.
Anybody here who has ever got a ride in an EVO knows that below 3000rpm they feel like a normal 4G93 1.8L 4 cylinder. But from 3500 to 7000rpm they explode to life and feel just like a 6A13TT, not some super special amazing engine ent?
The EVO 9 and X are rated at about 215kw and 400nm of torque, a 6A13TT will get over that amount just by increasing boost to 12-14psi with a cheap manual boost controller. Without changing the turbo a 4G63T is limited to about the same sort of power levels as a VR4.
As far as I can see the only advantage of going with a 4G63 is that you can buy manifolds and turbos that will let you get to 500hp or more straight off the shelf. However that will cost most folks an arm an a leg to buy an thats after paying for the EVO engine with your 1st born and a left kidney
So far the only people who have put 4G63's into 8G's are the Americans and thats because they cant find 6A13TT.