I've built both kinds of bows, and seen really nice examples of both glass and selfbows. I think making both kinds of bows is easy. I think making really nice bows of either kind takes an incredible amount of skill.
As others have mentioned, the skill set is different. To me, it's sorta like furniture making. I like making furniture, too. Glass bows would be like making traditional furniture, while selfbows are more like rustic furniture. One of the skills to making a traditional table and set of six chairs is how exactly you can reproduce each chair. It takes a skilled craftsman to turn each leg exactly the same, set the curve of the back exactly the same and make each peice fit flawlessly.
A rustic table and dining chairs, though, is a different animal. The skill is not in its sameness. The skill is in using wood found in nature, through dilligent search, that fits the function for which it is intended. After that, the skill is fitting each peice together so that the chair is as comfortable, or effective, as any traditional piece.
Both are easy to do, neither is easy to do well.
Stan