Are metal futon frames ok?

In my opinion, NO! It may be hard to see in the image, but there is almost no support in this type of futon frame. The "slats" aren't really slats at all. They're really metal bars that are spaced about six inches apart, thus they provide no support.

Now, there are some frames on the market that are metal and wood combo frames with flexible wood slats. Those frames are different. The spacing on wood slats is appropriate to provide support. Rounded metal bars only create pressure points, especially when they're too far apart.