If you find old, gray, skeletal, rock-hard wood, it's good to use whatever kind of tree it came from. Otherwise, you really need to know what sort of wood it is. Some woods rot quickly in water and spoil your water quality. Others have toxic saps.
The hardwood/softwood divide is not enough by itself; some softwoods, such as baldcypress, weathered red cedar, and white cedar are fine in tanks, while some hardwoods, such as willow and walnut, are not suitable for tanks. It is true that softwoods such as pine and spruce are totally unsuited to tanks, being toxic and prone to rapid decomposition.
DIY driftwood is great, but really does require a bit of tree knowledge. If you aren't sure, don't risk it.