Have been attacked plenty of kobold thieves and goblin snatchers.

Have had no ambushes.

Have been attacked 4 semi/megabeasts and several werecreatures.

Have only had one siege in 6 years despite 140 pop, >1M wealth, hard enemies difficulty setting, and being close to active goblins.

I did finally get a second siege after changing the difficulty settings for minimum number of raids between/before sieges to 0. The siege came the next season after changing the settings.