by "can not always" be relied upon.
if your Operating System is free of bugs, (viruses, spyware, safe from hackers, etc), and is working well, then System Restore should not present any problems.
However, System Restore is a powerful tool and should only be used when absolutely necessary. Best practices should be applied, ie;
* when installing a new program, create a retore point first, in case the program install fails for any reason.
* when making major changes to your OS, create a restore point first, eg, if upgrading your OS, backup your important files first, then create a restore point.
* If you ever find a virus, spyware or other malware, remember that it may have infected your restore files, so turn off System Restore, reboot into Safe Mode, re-scan with your anti-virus, anti-spyware or anti-adware utilities, then when the scan is complete and shows up clean, turn System Restore back on, and reboot into normal mode.
* If you can afford the disk space, allocate the maximum hd space allowable to restore files, (Control Panel > System, System Restore tab).
* Keep your OS clean of temp files, TIF's, (Temporary Internet Files), and empty the Recycle Bin.
* Perform regular Error Checking and Defrag scans, (once a month, or once every two months, depending how often you install/uninstall programs or save/delete files).
* Don't continuously install and uninstall programs. If you want to test programs, find an older machine to do the testing for you. Keep your main PC as neat and tidy as possible
There are options available, and I see that you have received other replies in the PC applications & utilities forum, but I have never found the need to use any other backup/restore utility, so I can't judge them.