Good Against: Nothing.
Bad Against: Everything!
Why use them? Scouting around regions only. Non-combat unit.
Advanced tactics Scouts are not a combat part of a mobile fleet (unless you enjoy lulz), but they are very important. Information is the most valuable commodity in Astro Empires, and scouts can get that information to you the fastest. They are the fastest unit in the game for a reason It is advisable to have 20 - 100 scouts in your fleet, depending on how often they are shot. In the words of Peppy Hare, "Just shoot it, Fox!"
While Scouts only have a power of 1 and an armour of 2 it IS possible to attack other units with them. You simply need vast numbers to have any effect. While this in NOT a sensible tactic it is good for a laugh if you want to try it. You can even take an undefended base with a single scout. That always gets laughs and a red face on the owner.