Funny you should mention the ladder - I am currently emailing this guy about his ladder
Not that I'm knocking ladders, I would probably use one if I could but I have Harken Batcars on my mast which obviates such ladders.
The problems I have thought about are:
The treads not being laterally contained and sort of slipping around the mast when a foot applies pressure to one side.
Then there is the main halyard being used to hoist the ladder so if you want to belay yourself as you go up, one could use the topping lift if you have one else you have to use a headsail halyard which is a bit of a problem because it comes out of the masthead sheaves on the wrong side and has to be fed around the rig (spreaders etc.) Not insurmountable but a hassle nevertheless
Then there is if you're going up the mast on your own (no-one holding the safety halyard), what do you clip your harness to? Because if the main halyard (or the ladder) parts, the ladder comes down on top of you after you've hit the deck although you probably won't notice.
Then there is the problem of what do you do if the main is up and you're sailing but lose a headsail halyard and have to go up? You have to stop the boat in a rolly sea, drop the main etc. When the main is up and you're sailing the boat (and mast) is way more stable for going up.
I may be sounding pedantic but I have had to do this sort of stuff when I was single handing across oceans and any one of these issues would have turned a simple climb (I had those enclosed steps) into a really daunting one.
Not trying to talk you out of a ladder because for someone who returns to a marina every night the ladder is a really good solution.