Chain tension
 

Would you say it's better to have a chain that's a bit slack than a bit too tight? (having it spot on is obvs better, I'm just curious)

Better too slack - but not hopping off the sprocket slack!

Too tight puts strain on the output bearing (front sprocket) - I've picked up a number of bikes in the past with busted cases around the sprocket because the owner had the chain too tight.

Co-incidentally both bikes were MZs (one an ETZ, the other a Supa 5) but different owners. The MZs didn't have a reputation for sprocket bearing problems (the early 4-speeds did for mains). The only thing I can think of was the chains were fully enclosed (a great system) and the owners couldn't see the chain and simply mis-adjusted them over their ownership.