Had my Nüobells for just over a month now. Extremely happy with my purchase.
Previously I have used a set of Bowflex 552 knockoffs ("pandemic special") that kept getting stuck and eventually one of them failed. No such thing with Nüobells - they are rock solid. I love how easy it is to adjust the weight, and how grippy the handles are.
Of course there is some concern with long term durability - there are some plastic parts that will see wear every time you remove/replace the dumbbells from/to the stand. However, they seem very beefy (again, compared to 552s) and I have a feeling they will last for years. Also, if you do misplace the dumbbell and it gets "stuck", it is very easy to to fix this by dialling down the weight and putting the weight plate back in the proper position. Overall, I feel like there is some tradeoff in durability vs ease of adjusting the weight compared to something like Ironmaster or Pepin dumbbells. For my use case, I am more than happy to make that tradeoff. If you want more information, I feel like Garage Gym Reviews on YouTube review Nüobells very fairly and accurately.
Regarding the seller, I have nothing but praise. I was a little hesitant ordering online based on a Facebook ad. However, Fitmass is listed as an official distributor on the manufacturer's website so that gave me some confidence. Shipping was fast, and customer service delightful.