According to Forbes, Apple Music Could Wreck Your iTunes Library [forbes.com]:
At its heart, Apple Music is a simple proposition. For your monthly subscription fee, Apple will offer you access to a library of over 30 million tracks. You can listen, explore, and discover to your heart’s content, and you can take that music with you wherever you go. But subscribing to Apple Music and making full use of the streaming service requires a sacrifice.
You have to hand over control of your iTunes music library to Apple and hope that Cupertino’s arrogance will preserve your music collection.
[...] The issue that is upsetting many Apple users [macrumors.com] is that moment when you turn on iCloud Music for the first time and your tracks are synced to the cloud. Apple’s methodology on this is not clear, but from reports and feedback from users across the internet, it appears that Apple’s view of metadata and what the ‘correct’ track is, will take precedence over your custom edits.
The Verge’s Chris Welch [theverge.com] highlights his preference of listening to early tracks from The Beatles in mono format (just as they were recorded) rather than the automatic matching services’ preferences for stereo versions. Support forums talk [apple.com] of collections approaching 20,00 songs becoming corrupted and full of duplicate entries, incorrect meta-data overwriting current entries, album art switched out to show the wrong albums, and more stories of personal pain.