For individual users, there are very few reasons to keep using Legacy. V10 has improved over time and is ready to use.
But in a business environment, there is a critical issue regarding the confidentiality of tags visibility. With V10, everyone see all tags. Within legacy, tags visibility depends on Notebooks/notes access permissions. Thousands of tags need to be checked and modified and nobody does it while it is working.
In the minor issues, there is no possibility to add color to a tag, and this is part of our way to use Evernote. Our workflow needs to change and everyone trained to it.
Evernote stays for me the best software for our usage and we are moving forward to migrate to V10.