Hmm, from my experience
The Ranger unit is a nasty little lethal, with good armor and HP, fires only on lethals, and does both AD and HD, slow unit, being ETA 5, and fires late though. Good for big battles or for fighting people who don't have an even spread of AD and HD.
The Harrier unit is useful in a different way. It has a bit of armor, but not much, fires after the usual armor strippers, has a little bit of HP to keep strikers/RPGs from PWNing them horribly, it also does a little bit of HP damage, so it can kill some routes that RPGs/Strikers can't w/o early firing HP. Its low ETA makes it great for rushing, as its difficult to defend against, need a lot of them to effectively kill whatever your rushing though.
Rangers can be killed by strikers, RPG (if its an even spread between RPG/Sniper) Robots (If Ranger heavy), and TL.
They are usually flakked through by poms, but F-117s can prevent that.
Ofc, as with all routes, what it can actually fight is heavily dependent upon your unit setup. How you want to play will determine what you do.