Almost none.
The scanpst utility is built by the engineers who were architects of the PST/OST file formats. IT does the best job repairing corrupt files of that type. In order to actually remove all the errors you may have to run the utility several passes to get all the errors.
You also archive emails into separate PST files. Large PSTs files become problematical for Outlook. After you archive and/or delete old emails you should run the compression option on the PST files to compress it's size. It available when you bring up the properties for an Outlook Data File via the mail control panel icon or in accounts within Outlook.
If you're having issues with Outlook freezing, crashing or otherwise not receiving/sending emails it may not be the PST/OST files to blame. The profile (registry settings/user configuration) for Outlook may be corrupted. You can setup a new one using the control panel mail icon. You'll have to reenter all your email account settings, RSS feed details and so on. You can use the mail control panel dialog to backup profiles to have when and if they corrupt.