We ran into some weird problems on our Windows development machines when using symlinks as deploy-strategy.
For some reasons PHP refuses to include the symlinked file when it has a relative path that is quite long. The file can be opened without problems in explorer-context.
This causes the mg Autoloader to fail and classes can't be loaded. A simple test file that tries to include the symlink fails aswell.

Source: D:\www\mg.testenvironment\vendor\connect20\DerModPro_BasePrice\app\code\community\DerModPro\BasePrice\Model\Entity\Resource\Eav\Attribute\Product\Amount.php Symlink (as output from cmd dir): ..\..\..\..\..\..\..\..\..\..\..\..\vendor\connect20\DerModPro_BasePrice\app\code\community\DerModPro\BasePrice\Model\Entity\Resource\Eav\Attribute\Product\Amount.php 

The resulting path is longer than 260 chars, which is by default the maximum path-length windows can handle. But I can only guess that this is the reason.
The depths of magento's paths give a quite high change hitting this limit.

When you change the symlink to an absolute path - manually in cmd or hacking in Symlink.php+redeploy - the include/Autoloader works fine but this would drop some flexibility.