Off the top of my head: You tie your data to one provider instead of simply physical media, you lose discoverability (will your kids browse your album choices 30 years from now?), exporting/importing quality is at the mercy of the provider (for example Amazon dropped several dozen mp3s when I migrated a few gigs to Google), and of course you place your data at the mercy of a business, (is any tech company eternal and/or always interested in providing cloud services?)
The cloud is way overdone, after giving a variety of services a go over the last few years, I'm actually pulling back.