Yes ok, in this case the drawing is the problem (yes I know, I contradict myself but in the previous case, I knew how to draw and that was not my problem).So what is the problem with using billboards then?
The problem with billboard is that I would like to draw the bar myself, so be able to give it a specific color (that can change depending on the health of the unit) or have a gradient in the bar (from green to red). Both solutions are not really nice with billboards: the first one doesn't work because you need to specify a texture that will be rendered for the billboard. Ok, I can still go for one single color (or maybe three) for all life bars but that won't be very nice.
The second solution: ok, I could have a gradient in my texture, but then, when I resize my bar (because of life loss) the texture will still be completely fitted inside the bar (so, the complete gradient will be there, from green to red). Hummm, very ugly also.
One option could be to go for two billboards: one with the gradient and one completely black to mask the first one (they need to be drawn in that order). I just thought at this solution now. It is not very elegant but if this is the only solution....