Its reliability is close to 100%... the only chance of it being wrong is if your boot sector is infected by a virus- in that case it could report a defective drive even if it was basically healthy but capped by the MBR pest.
Since this is Windows app, and "something" may act up though, one way or another, you can repeat the test with just your UI (browser) open, and everything else (antivirus, firewall, tray utilities etc) shut down.