Small computer fighters are chewed up in fleet combat operations. The computer difference to fleet computers means they can struggle to hit and penetrate the defenses of combat ships. When they do the weapons carried don't do much, especially if the ship is armoured.
Against civilian ships however, small computer fighters can be very effective. Civilian ships typically have a computer sized to their jump needs (ie: close to the fighters computer size), limited agility, limited defensive fire and no armour. Fighters also get the maximum small ship modifier.
Fleet fighters are what most in this thread are talking about when referring to "unhittable" fighters. Fleet fighters carry the best computer available for that tech level and also max out on agility. In effect at the bleeding edge of technology, the two opposing fighters cannot hit each other. This is not uncommon, for example armoured knights fighting to exhaustion or M1 Abrams tanks unable to defeat its own armour. Tech advances normally sort this out. Or tech disparity between opponents may mean this is not a concern (unless you sold them your advanced tech).
At TL11-12, fighter fleets have the edge over the same TL BB fleets, just as in 1942 Brown Shoes came to the fore. At TL14-15 however computers cost too much for Fleet fighters and BB fleets have the edge. By cost to much, I mean, yes you can do it, but you get better better bang for your credit elsewhere.
Are fighters "unhitable"? No. The main anti-fighter weapon is lots of factor 9 missile bays. Don't use spinals on fighters, save spinals for capital ships. Why? Because you only get one shot at full effectiveness, during that same round your ship takes fire and all your weapon systems take damage. This weapon scrubbing effect is what fighters are really good at. The spinals in a flighter fleet come out to play only when the ability to scrub them is much reduced.
At TL11-12, fighters can overwhelm missile bay defences. However as the tech level increases, fleet fighter computers get much more expensive (plus the power plant to run them), while factor 9 missile bays stay the same price or get cheaper and smaller.
The "unhitable" fighter "screening" the rest of the fleet. This only works if you know the fighter is in fact unhitable. For example, there are no factor 9 missle bays left in the other fleet. The consequence of getting this wrong is fairly bad. If the figter is taken out, the enemy fleet gets a free round of firing (no return fire) on your reserve. There are a lot of historical examples of opposing forces leaving pickets to screen the army while the respective armies lick their wounds and reorganise. And historically an army tended to get punished if their pickets were rolled up.
None of this takes away from house-ruling the bits you don't like. For example, no fleet pickets or abstracting away what it takes to overwhelm them to gain a free round of fire. As another example, I like the fighter squadron rules in JTAS (the journal number escapes me). It is worth keeping in mind though that in threads like this, there are often contradictory proposals to both strengthen and weaken fighters because they are both too powerful and at the same time cannot do anything.
If you want to experiment with fighters, fighters at TL11-12 are the most fun. TL13 is marginal. At TL15, Imperial fighters are typically not facing TL15 opponents, meaning they may not need factor 9 computers to be effective in fleet engagements. And of course to keep the Imperium in line it is fairly cheap and fast to build or replace TL15 fighters capable of matching the Imperial average TL fighter.
Just food for thought.
