I've only had to defend Oberland station and it was a few raiders, one did have a missle launcher though. I took her out first and the rest of the fight was fine. Like 5-6 in total.

sat on my tower and watched the raiders get cut down by my Turrets.

Well this just happened last night to me.

Early game (still 7th level) and I befriended Abernathy farm West of the Red Rocket. Hadn't even gone to Concord yet so I did that and put my suit up at RR, then back to Concord to loot the place. My Pipboy says the farm needs some defense so I went over there to make some turrets. I made three and went inside to talk to the mother. While I was talking to her gunfire broke out outside. I had to leave the dialog (which I've never been able to start up again dang it) and run outside. There were three Gunner raiders attacking the daughter and father. I had set the turrets at the corners of the farm, thinking it gave good coverage for anyone approaching, but apparently the raiders spawned right beside the house. Anyway these Gunner raiders were a lot tougher than the ones I just killed in Concord. But I got them all.

I'm concerned that it happened so quickly. I thought three turrets would be more than enough to discourage raiders from hitting a tiny farm, but I guess not. So what kind of defenses should I get?

Lackluster. Three raiders attacked one of my expansions that I hadn't even touched yet. I'm focusing on Sanctuary, so I'm excited to see those turrets do some work.

I'd say at least half of my 'defend' quests go like this: I show up, there's nobody there. Everyone's just walking around, then waaaaay off in the distance I can sometime VATS target like a raider or molerat or something like a mile away. Then the entire settlement freaks out and charges after this one guy. The guy dies and everybody thanks me.

I've only seen 2 actual attacks - raiders at the northern radio tower place and synths at the robot farm (and I was friendly w/ synths at the time)

I laughed evily while my settlers got wiped out, they keep annoying me with their constant low happyness levels, I don't know how to make it go higher game.