Granted, this was about 1.5 years ago, so a lot has probably changed, but when a fellow engineer friend and I decided to switch to Matrix, we have a lot of trouble getting the end-to-end encryption working. It required a lot of finagling and sharing of keys and was pretty annoying.

Being engineers, we did figure everything out, but I couldn't see any non-technical users going through the headache, particularly when Signal is just "install and you're done".