Hello,
I am planning to install the free version of nextcloud in the linux VPS of one of my clients
and was wondering how reliable is the windows client.
Now that you are mentioning windows… it’s getting a bit confusing here 
Trying to clarify it a bit: you want to install NC-server to Linux subsystem of win?
Or do you just want to find out how reliable NC-desktop-client for Win is?
Installing NC-server on a Linux subsystem of a Windows installation is neither recommended not supported. Judging from what I read on the forum it’s possible though but it may come with problems and you are mostly on your own with it.
NC-desktop-client for win seems to be pretty reliable and easy to install.
Whatever you need or do or need to do - take backups