Every version of Marble Blast Ultra on our download page is safe.
Versions 1.6 Beta 4 through 1.6.1 contain a faulty Server Monitor that has the potential to cause a memory leak after around three days of usage without closing/restarting it. The descriptions of these MBU PC Edition versions reflect this. The issue is not extremely dangerous and the issue is unlikely to arise, but just to be safe, make sure to uncheck the Server Monitor tickbox in the installer if you install one of those versions. This problem was fixed in version 1.6.2 and up, and the latest stable version has a safe, working Server Monitor.
Sometimes, versions 0.2.0 and down accidentally deployed a Skodna.GameHack virus. Any files on our download page have been fixed, have had that virus removed, and are safe.
MBU PC Edition is completely free. It will not expire or stop working after 30 days. It does not have any locked levels. The astrolabe is the limit. It may seem too good to be true, but it's all true and it's definitely all good!