I've been using them in private work (i.e. renovations on my own place and helping friends) for about 5 years now and never had a single problem. They're not listed for stranded conductors, so then I still use screw-type connectors. Wago does offer connectors with release levers for stranded wire, but they're larger and considerably more expensive.
Wago sells a special antioxidant paste for aluminum wiring (Wago AluPlus) but I think in Germany it is not even required to be used as long as any generic antioxidant is used. I never heard about any trouble with that combination either, and in former Eastern Germany aluminum wiring is extremely common (it was used almost exclusively until the early 1990s and you can still find NOS rolls left over).
I assume in the US it's mainly a listing issue - I think there aren't any UL-listed Al connectors except for the Copalum crimps, are there any?
