Evil Google? Looks like a duck.

On the con side, you’re also opening up your website to changes which you never specifically approved (other than all the general legal approvals you give a site by agreeing to their Terms of Service). If one day you decide that all the interface changes, or ads YouTube rolls out, are becoming too distracting for your visitors, the burden of replacing all embedded code and finding another host (if you kept your video files in the first place!) lies on you.