As I never do saveload - also not when in the campaign environment - I would like to hear some experience.
Is there a way to take down a harvester with only human tech for instance. Generally, I attack anything larger than a fighter only late in the campaign when I have starchaser already, which can take it down safely (with 2 particle beams, 1 laser and 2 alien warefare suits, no armor). Recently I downed a fighter with a saracen (2 particle beams, 1 alien warefare suit, 1 targeting computer, no armor) and survived with 4% hitpoints or so. When I already was that low I decided that only killing the fighter would safe me now, as I would probably have been hit again while trying to flee, and it worked. So this was too risky, I can't afford losing the 50000 credits or so which this ship is worth. This was in 2.6, where saracen has less hitpoints than in 2.5. IIRC a fighter can be taken down by a stilletto with 3 lasers too, don't recall about the electronics but at least 1 alien warefare suite seems mandatory, and no amor. As armor makes the ships slower, I often don't use it.
Human techs only are insufficient to attack anything armed as to my knowledge, so only scouts and supply ships are possible with those, which makes the human tech air rockets useless.
EDIT
IIRC I also once, in despair, destroyed a harvester which was chasing my firebird dropship with all the good soldiers on board using 2 stilettos and a ground SAM site. The stilettos were IIRC armed with 3 lasers each and no armor, don't know about the electronics. The harvester ignored the stilettos and continued chasing the fleeing firebird, which made victory possible. As stilettos are slower than harvesters, which appear very soon, I normally prefer saracens instead, as stilettos cannot flee from a harvester chasing them.