I can only speak to the first part of the Norte, which I walked this spring before turning south onto the Primitivo. I can't compare it relative to how well other routes are marked.
As others have indicated, I found it pretty well-marked, most of the time, so for the most part just following the arrows was enough for me, but not always. Some of the times where I wandered off-route it was relatively easy to correct, other times not so much. I was often walking alone, the distances each day were long and with significant elevation gain and it was my first Camino. I also find it particularly hard to navigate cities in the best of circumstances, but found it particularly hard to find the arrows in certain cities along the Norte.
I was glad to have the
Wise Pilgrim app to confirm I was on the correct route, and Google Maps to find my lodgings once I arrived in at my destination. I also used Google Maps from time to time to check just how far away my destination was because often my route planner or the markers indicated the day would be X km, and it was often X x 120%-150%, and while Google Maps wasn't entirely accurate as to how much further I had to walk on the Camino (and instead was providing distances based on a road route), it at least gave me some idea how much further to walk when my information wasn't matching the actual number of kilometers I was walking.
Over the course of my 6 weeks, I managed to take a number of wrong turns, none of them too dramatic, and of course I eventually found my way back, but when you're super tired already, walking 30 minutes off-piste means adding an extra hour to your walk as you retrace your steps (and on the Norte, likely some significant extra elevation gain), although it does provide a good occasion to practice that self compassion and not allow yourself to get angry about it. So if markings weren't clear at transition points, or I had walked for a distance without seeing an arrow or other marker, I would just check
Wise Pilgrim to see if I had wandered or was indeed on the right path. There were definitely places where it was easy to miss the markings, although fortunately that was the exception and not the rule.
One other thing I learned fairly quickly though, was do not just assume that the pilgrims in front of you are going in the "right" direction and follow them! They could be headed somewhere else, or they could have taken a wrong turn. I subsequently heard many stories of someone inadvertently leading a pack astray, only to have to reverse course an hour or so later after the mistake was realized. I'm glad I wasn't responsible for any of those adventures, although maybe they all bonded on the return trip to find the route and had a great experience notwithstanding the extra hours of walking. Also, I definitely experienced situations where others called me back before I went off in the wrong direction, and times when we all stood puzzled at a cross-road trying to figure out which way to go and
Wise Pilgrim got us going in the right direction.
For someone for whom this is a first Camino or who doesn't want to experience whatever magic wandering off route can provide, I definitely recommend using one of the apps just to check your route from time to time. I get it that there are others who will want to just wing it and live that experience, but I'm not sure that's for everyone. I should also note that
Wise Pilgrim wasn't perfect either - there was one spot in particular on the way to Ribadesella where the path was not well marked, and
Wise Pilgrim showed crossing a stream where it was not possible and required 30 minutes of backtracking and the fortunate appearance of a local to help us find the way across. That was my 25 mile day, and as beautiful as that alternate route was, I would have been happier at the end if it had been 1-2 hours shorter without losing the trail.