They work ok in areas that aren't particularly congested. Even then, many of these shared bike/pedestrian ways are just packed full of little things that slow cyclists down in a way a cycle lane wouldn't. They're also poorly maintained as compared to the street that gets re-paved every so often while they just fill the worst holes & cracks on the sidewalk.

That's why some enthusiasts in .fi choose to cycle in traffic (even though that's not really legal if there's a bike path on the right).