I have a US Turbo Express, a japanese Duo, and I used to have an American Duo. The Neo Flash card worked great on all of them. When you write to the card, there is an option to select "PCE" or "TG16" mode, so if you go between a US and Japanese system you have to rewrite the card.
On the American system, you can burn any rom to it and it will work. On the Japanese system, there are a few US roms that won't work, but that's generally not an issue since most US games havea Japanese release that is already in English anyway. The only US game that I've run into so far that absolutely will not work is Cadash.