Is kydex comfortable against the body?
While kydex is a "plastic", the thought of having a plastic holster against one's body isn't going to be comfortable. That is true depending on few key factors. Kydex holster with a high quality edge finish, size, and shape all play part into it's comfort level. If an inside the waist band holster is large and bulky, it won't fit right and rub. Also if the edges of the holsters aren't smooth to a glossy or matte finish with the proper tools, then those edges are going to scrap and rub your skin to the point were you'll hate it. A quality kydex holster for inside the waist with proper edge finishing, shape, and size will fit with ease and comfort just as good if not better in some cases then leather.
With kydex being water proof it doesn't absorb sweat like leather. There for heat rash and chaffing will never be an issue