The con for a switch over from stern gland to dripless is cost in my view.
It is a big job in a really confined space that must be done right.
I have a stern glad that leaked too much. On inspection it was found that the shaft was gone.
I know that the gland material should wear away before the stainless shaft, but I was up for a new shaft.
After removing the shaft, (no small task) and fitting a new one I was already to fit a dripless, but there was just not enough room.
So I am still on a stern gland that (after what I had before) lets almost no water into the bilge.

So my take is, if you have to remove the engine for some reason go for a drippless.
Otherwise, use good quality gland material and keep on top of your drips.

hgary