Mostly be rushing at the learning curve again. Always something new when I come back.
I've dealt with GCSes eating everyone near the chasms, everything/one I embarked with burning due to volcano imps, traps destroying every caravan, learning how newfangled hospitals worked, a plague of giant "trees" (real trees are 1 tile, and don't lay eggs) causing dwarves to climb the the top and starve to death, wereanimals destroying everyone, and now these visitors.
They were all some sort of fighter, a few legendary, so I assumed they'd help with defense. What they actually did was drink my booze and increase the population over 50 within the first year (way before my military got started), which got me raided by a necromancer with like 6-8 zombies. They *did* get my population back down, but also my entire remaining population seemed to be the zombies that attacked. I assume the correct solution is "be picky about why you accept" or "maybe don't have 5x as many visitors as population, if you keep hitting population benchmarks for invasions before you're ready" or other undwarfy sanity; but I'm pretty sure I just need to embark with the makings for a magma pumpstack, 5 miners, and plenty of booze (also, some for the dwarves). Or some sort of hydraulic visitor-cannon.
Not enough random cave-ins or tantrums, though.