I think being specific is great and letting them know what they did well is good. Phrasing it more in line with we found someone with a bit more experience in XYZ can help and even making it seem like if they take some months to brush up on... lets say python... they could be a stronger fit down the road. Keep it honest, but avoid specifics.Too many people can nitpick rejection but if you imply there was someone just a better fit and give them some points on what makes someone (even if that someone isn't identified yet just an ideal profile) will help the candidate in the long run. Also, if you've gotten them deep in process like an on-site CALL THEM. 4 hour interviews and a technical challenge, you can spend some time giving them a ring. Especially Sr People don't make them feel like you wasted your time and sent a canned rejection.