I would have to agree with you there.

how well defended are pirate bases?

A pirate base will generally have a lot of turrets (around 20 or so) as well a respectable compliment of ships hanging around (mostly Pirate Cutthroats and Corsairs, fairly beefy units, but also some Reapers and Pillagers). Overall though you can expect a good 30-40 pirate ships to respond to you when first enter the Gravity Well, and then the 20 or so Turrets stationed in a circle around the base. There can be more ships if you attack as a pirate raid is embarking, but all of the raids I've seen have been content on leaving their base and going to their target and not attacking my force.

The turrets are basically TEC defense turrets with good armor. They're pretty decent against ships but are easy to engage at range with strike craft.

The pirate ships as individuals aren't all that threatening. None have any shields except for the Cutthroat (and it only has 200 shield hit points). Basically all of their units have between 1,000 and 1,500 hull points, so around that of a Cruiser, although I'm unsure of their classification, most seem to fall somewhere around the Frigate or lighter Cruiser level. They're only really a threat as they mob you right as you enter the well, things can quickly become just a furball with units all over.

It's a formidable defense early game, but later in game it isn't that much to take out. As Advent I've not had much of a problem going in with just a small number of capital ships and wiping them out. The only thing to really remember though is that the Pirates never retreat, they attack until the last one is dead. I only really wipe out the Pirates though if they're in the way, since their constant suicidal attacks later into a game make for great experience for capital ships.

Pirates also get no shield mitigation, which makes them pitifully easy to focus fire into oblivion.