Crossbows were heavily backed wood with sinew and best with a horn belly...or the prods were steel. The stronger ones (we're talking about hundreds of pounds) had to be cocked by a windless (or twin windlesses) and were very expensive and very slow to reload. Also while they could be very accurate, they ddn't put enough fire power (wood power?) in the air and because they were sightless, pretty well couldn't be elevated enough to accurately aim at distant targets. The English longbow was not as powerful, but could put out dozens of arrows in the time it took to reload. Every time armies of crossbowmen faced armies of archers, the crossbowmen lost.