The only time I have experienced it was when doing mutual with another guy and he did that to me. I didn't know he was intending to and didn't find it very comfortable either. Certainly the feeling of ejaculating is, for me, much better.
I also don't see what the big deal is about cleaning up. I quite like feeling it land on my skin so I let it go on my chest/belly and then just wipe it off with a towell. The chest/belly is quite a big area so would be able to take a pretty big load before being overwhelmed.
If you prefer not to get it on you there are so many other ways you can avoid a big clean-up job. You could aim into a towel or a pair of underwear - if you point you dock down onto one of these there should be no chance of overshooting. If you have a sink in your room you can cum into that, you can cum in a glass and empty it into the sink, you can cum into the shower etc.
As for how safe it is, semen in your bladder should not be a big problem but I don't know if the increase in pressure in the part of your urethra with the semen is in healthy. I don't know of any medical literature confirming problems but given thatr I don't find any pleasure in this technique it just seems better to me to avoid it.
