I teach beginners to do koshinage all the time. I load them up, walk around with them, and let them down extremely gently. I do it a lot when teaching that technique so they get the idea that lining up your structure is more important than the uke going splat.
Let's face it. There is a lot of aikido out there that rush through the beginning and middle of techniques just to get the big dramatic/ego-satisfying ending. Koshinage tends to engender that issue to a more hyperbolic degree which works against the more desireable goal of unification with both people add to the overall movement.
My opinion of koshinage is that there are 2 spirals at work going on at the same time. One of the spirals is going parallel to the floor around nage, and the other is going perpendicular to the floor (up near nage, out and over and down uke, and back up nage).
The first toruble is getting the unification happening to some degree.
The next trouble with beginners is getting them to stand with enough basic structure to support someone else on their hip (assuming they are not going to master the 2 spirals right away).
The last trouble with many aikidoka is to get them to not completely finish spiral 1 before starting spiral 2. You have to be able to walk AND chew gum at the same time.
Really, these issues are the main issue with most of what I see being practiced at the "ha" level of aikido. Koshinage just makes it a bit more obvious.
There are some koshinages I avoid teaching. There is 1 in particular that when I take ukemi I find myself having to change how I'm holding on to nage 3 times to stay safe. I can't teach that safely to someone who doesn't have pretty good body skills built up. I also avoid teaching shihonage koshinage as I have never been able to determine any real value in it at all. Shihonage is interesting enough; why add extra danger for new people for lessons they could have gotten much more safely from other throws...