crank length question

My sun 28" has 178mm cranks on it, and it seems like it makes it go slow. So I got my 20" unicycle cranks which are 150mm and put them on. Doing that it made it go faster. So what I’m think is the longer the cranks the more torque you have, and the shorter the more speed you can get. Also if I were to get shorter cranks would it go faster (or more likely to hurt myself).

There is a control issue with extremely short cranks, 89mms on my 24" is great for commuting(decent cruising pace, and I can put the uni under my desk!) but there are certain situations where it becomes very difficult to recover due to the low amount of torque, especially hitting a large obstacle in the dark:p
I call these the “I have short crank arms” dismounts, and they don’t really occur with longer crank arms than the very extremes.

Unless you’re very slow I doubt shorter cranks will lead to injury, however.