Without subscription nothing changes. It stays as it always was. You still have a server with all modules, all options to install other applications and all the updates that will be available for NethServer.
What would you consider unstable updates? If you deploy updates on a mission critical server, you are always playing with fire. It is good practice to test updates before rolling them out on live servers.
The reason updates are just fine 99 out of 100 times is because there is a great team of developers, testers and Quality testers in this community.
So, as always, if you encounter any problems with your server or with an update, open a topic and I am sure it will be fixed. Every help with this process is welcome, so if you have some spare time, please do help out testing.
If you need more assurance, do get one of the subscriptions. You will get access to the (delayed) repository for subscription servers. These updates are not different from the default updates, but just more extensively tested before they are released.
Security updates are always pushed as soon they are available.