I've finished a few hackberry flatbows in the last year and I agree they seem to better suited to the flatbow design. I find the wood very pleasing to pull a spokeshave through...feels a bit rubbery, but the wood has a very consistent grain and I can take really big bites with the spokeshave even when the wood is really well cured. The wood comes out with a pale buttery yellow color, looks great with s simple oil finish, but you can also get it to take stain really well. I have put some nice recurves to the tips of two of the bows, it takes dry heat well, steam works pretty well, too.
One nice side benefit is that the wood smells like hot buttered popcorn when worked.
As far as making a warbow...go for it. Give it a try, start a bit wider than you think you might need just to be on the safe side.