Kicking is very hard on uke.
The defense for most kicks is very much the same as defending against low sword cuts (If you can kick over the top of your head like a shomen sword cut then you are "unique") Nage can easily control the distance to uke by steping. The knees and hips are much more powerful than the shoulder and elbow, hence the danger of a well placed kick, but the are no where near as versitle. I have occasionally practised kicking techniques on the mat and I will report that you spend a lot to time on the ground as uke. It does not take as much "aikido" as you would think to take uke balance when he by necessity off balance to kick (100% of weight on one foot). Picture virtually any classic aikido techique and apply it to this big rigid low-slung arm/leg and boom uke is on his back very fast. Take great care because rolling and falling are difficult.