Private servers can disappear overnight due to lack of funding, developer burnout, or legal take-down notices. Do not invest real money expecting long-term data permanence.
For the uninitiated, a private server is an created and run by third-party developers, not the original game company. They are built by reverse-engineering or using leaked official code to offer a version of the game that operates independently of the official network.
Look for servers with active, populated Discord channels. A transparent staff team that communicates updates frequently is a sign of a healthy server.