Only as good as the data in the system and sometimes that data source doesn’t seem to know about low bridges and narrow roads. Need to be wary, do a bit of research and read the road ahead. The worse thing you can do is blindly follow the Sat Nav which some folks seem to do. I’ve encountered quite a few Motorhomers and tuggers in my part of Cornwall who’ve got themselves into difficulties and they all say something like “my Sat Nav didn’t warn me”!Why, are they no good?
If you programme width, height,weight and know what width you are with mirrors folded or not, and use in conjunction with a good pre journey map read, are they useless?