Anyone used these?

Could avoid me having to buy various new cogs

https://www.sjscycles.co.uk/chains/gusset-slink-12-link-cranked-link-12×332-pitch/

Won’t that play badly with a thick/thin chainring?

I’m not using one, just simple single-speed chainrings, but that’s good point I didn’t think of.

I was mostly thinking about durability

Used one for a 1/8″ chain once on a SS trailstar, it was almost the perfect length with the wheel slammed right to the back of the dropouts. Chain went slightly too baggy after a few rides and so the wheel had to be moved accordingly.

They can work well enough if you’re on the boundary of the “perfect gear” but no guarantees.

I’d’ve thought if you’re looking for a magic chain length, then there won’t be enough slack to necessitate a narrow-wide chainring.