All scanning tools are susceptible to glitches, bugs and false positive
detections. Even major anti-virus vendors are not immune to such issues either. In most cases when these problems occur, the anti-virus vendors and security tool developers take quick action to correct the problem and provide support to those users who have been affected.
Further, no single product is 100% foolproof
and can detect and remove all threats at any given time. Just because one anti-virus detected threats that another missed, does not mean its more effective. The security community is in a constant state of change as new infections appear and it takes time for them to be reported, samples collected, analyzed, and tested by anti-vendors. Anti-virus boot cd's are no exception so you will get various opinions as to which one is best.
Here is a list/review of various free Rescue disks which are available: