the HGs can be done in the car. but the failure rate is higher if you do it that way.
if you do it in the car:
you have to unbolt the engine mounts and dog bone, jack up the engine and push it to the side to get the head off.
you have to catch all the bucket s and shims when you remove the cams or they will fall on the floor. then you have to figure out which on went where.
you have to clean the block while bending over under the hood.
you have to been over the whole time you are working on it.
if you pull the engine:
you can put the engine on a stand or work bench.
you can turn it on its side when you remove the cams so the buckets and shims do not fall out and get mixed up.
once the head bolts are out, you can carefully reinstall the cams so the shims do not fall out while cleaning the head.
you don't have to bend over while cleaning the block.
the guys who are experienced at this can pull an engine in less than 2 hours. i met one guy who says he can pull the engine, replace the gaskets and re-install the engine in a long day.
but you do need a lift to do it. and a hard floor to work on.