I don't trust the velcro on them to use them on fully flighted birds. I have one, but it's a bit big for Cookie, and Bailee is even smaller than her. She really really hates wearing it, and it's a bit of a pain in the butt to put on. Then i'm nervous when she's outside in it because i think the velcro won't hold. I stopped using it ages ago, hehe.
A better harness that's supposed to be really good for small birds like tiels is the Aviator Harness. If they weren't so expensive here i wouldn't mind trying one out. They're supposed to be more comfortable and less restrictive.