[The AI storyteller seriously overestimates the turrets and sends raider parties that are just too much. I remember I got 10 turrets down and I had about 5 colonists. They sent a raider party of 11 raiders. Needless to say the turrets couldn't do crap against the snipers and my measly 5 colonists were no match for a 10+ group of pirates, space marines, assassins and the like.
Whether or not they overestimate turrets depends on interpretation, which is part of the problem. If you just build 10 turrets around your entrance, with maybe sandbags for a little bit of cover, then a single sniper raider will means a very bad day for you (unless your own snipers manage to take him out quickly). On the other hand, if you build your turrets in strategic locations, such that raiders have to come within range of your turrets to get a sight on them, then 10 turrets can stave off a very, very large raid. Especially if you have builders on standby to repair damage, and use your colonists as well as the turrets. 5 colonists + 10 turrets vs. 11 raiders is a cake walk if done 'right.'
Doing it 'right' at this stage of the game isn't always fun, though ('right' for now means a funnel, a waffle, and turret placement strategy can vary). So like I said, I just decided to stop using turrets altogether.
tl;dr: turrets are overrated if they're used normally, but they're completely overpowered if you employ cheesy tactics. Tynan is well aware of the issue, so it should be fixed or improved eventually. I think a lot will change once he figures out how to mitigate waffling and funneling, though.