I walked the Primitivo lady May, I think I was about 3 or 4 days behind alaskadiver. I had no problems at all, walking in trail runners. There was one muddy stretch, that was highlighted in the guide, but I was able to hop around the worst bits one way or another. I saw many people using the road in preference for that section.
Deciding on footwear is the most difficult packing question of all for me. I start by reviewing past year weather, using wunderground, then, in the couple of weeks before I leave, I become obsessed with the BBC weather forecast map for northern Spain. I’m watching the bands of cloud, trying to decide how serious the rain is, whether there is time for the worst to have dried up etc. I make my final decision on what footwear to take at the last possible moment, when I have a rough idea as to what the weather might be like over the next 10 days. I know it won’t be 100% accurate, but at least I will know whether to expect lots of rain or lots of hot sunshine. That approach has worked well for me for all four caminos that I have walked in May.
I’ve also been lucky, in that I’ve always been able to walk in my preferred footwear, ie trail runners. But I would not hesitate to take my ‘proper’ walking shoes that I wear in the Lake District in the UK, if it looked like the conditions suggested otherwise.