Besides killing the application and making it unavailable to the user, this can break some types of applications in a more serious way.
It is the easiest method because it doesn't require you to partition your sdcard and root privilige. First disadvantage is that application files are just stored on the sdcard's main FAT partition. In order for the application to consistently behave as expected, developer should not allow the application to be installed on the external storage if it uses any of the following features, due to the cited consequences when the external storage is unmounted: Widgets, Services, Alarm Services, Live Wallpapers, Live Folders, Account Managers, Sync Adapters, Broadcast Receivers listening for "boot completed". So second disadvantage is that not all apps can be moved with native apps2sd method because of above reason. It's not working "out-of-the-box" as native apps2sd, you need to root your device and create a second partition on your SD card yourself. Or you can use a computer to create it;on Windows there are some free partition manager applications such as "Mini Tool Partition Wizard Home Edition", "Ease US Partition Master Home" etc. Thus each app you install will still have some data on the internal storage so you can still potentially fill up your internal storage even if you are moving all of your apps over. And, disadvantage is that, obviously, it requires root privilege and a second partition on your SD card. If you have a custom recovery installed (Clockwork Mod or Amon Ra) on your phone you can use it to create a second partition on your SD card. But when I install a new app I see reduction on the internal storage even if the app is linked. Link2SD does not link application's private data files that are located in /data/data directory, they remain in the internal storage.