Compressed air is safe as houses - cant rememeber the amount of times ive used that method to swap valve springs, retainers and stem seals
mmmm - I've even seen it done - but it would take a leap of faith for me to go for it, on a known good engine I might risk it, on a clapped out 20 year old unknown, I just can't bring myself to do it.
Problems I envisage ....
Oil that is sealing piston rings gradually getting pushed out and compressor not too happy about keeping up with air flow.
Stupid mistake by operator (I have plenty of practice with these)
Act of god (usually the case when working on cars - sometimes called sod's law)
The rope trick seems fairly idiot-proof (as long as I don't crank the engine when its holding a valve up) - unfortunately you can't do both rope and air.