I have not heard of OpenSea before either, but after checking it out and attempting to list domain on it, it's legit - just not well known. Yes, unstoppabledomains needs a little work on making it more user friendly, but the idea behind it is on point, as my point of view. When all said and done, The people that take 1 look at a wallet address, and its enough to put them over the edge, will be able to get drawn back in. Just like our internet, we used to have to type out an actual IP address sometimes. It evolved into all these domains. The same will happen with crypto addresses. "Yeah just send it to me at thedrop.crypto".
As far as it being a domain, I worked for GoDaddy for 3 years, and the .crypto domains do 3 things. 1) It acts as a wallet (send all types of crypto to domain name), 2) It allows you to actually use it to build a website on it, just point the DNS to the webhosting. These are uncencorable websites and domains, not on somebodys server and not related to ICANN. And 3) I have read that they allow you to purchase crypto with them, without the fees of an exchange.