Re: aikido vs jiu-jitsu
Dont overlook the fact that if you've used kotagaeshi to put uke on his back...HE is extremely vulnerable to an arm bar himself.
Add a stamp to the face first, step over and sit straight back into the arm bar and you start to get some nice'n'nasty technique.
But don't get overly fixated on the excellent finishing hold. It is fine for competition but not recommended for any environment where there are no guarantees that you have only a single assailant.
At the same time ...a kotagaeshi that is loose enough to allow uke to flip is great for aiki demonstrations. But has anyone seen one in UFC?
I'm sure its fine in context but for a more realistic view a practical kotagaeshi would have to take uke straight to the floor with a fair degree of wrist crush to make it effective.
IMHO of course.