Just a few issues:
- Thumbnails and images don't upload correctly.
- The user sessions don't work well with the rest of Joomla.
- Various error and warning message may be encountered prior to patching both Joomla and VirtueMart core files.
- Very limited options for shipping
PHP and MySQL version seem to be a factor on some servers. Not sure how there is such a high rating for this component or positive reviews. Perhaps some users didn't fully configure and execute VM prior to rating the product.
If you weren't running a complex or large shopping cart, then this might work for you but I doubt it.