Having your own client gives the user a good UX. You can't provide a seamless login, server switching experience, and whatever else they offer with a widely use

Having your own client gives the user a good UX. You can't provide a seamless login, server switching experience, and whatever else they offer with a widely used open source client (without forking at least). But you don't have to use their client. Most VPN providers (looks like Nord included) allow you to connect with any client that's compatible with one of their protocols. I use a different VPN provider with the official WireGuard client, even though they have their own company-made client.