Many sections indeed are suitable for most bikes - in some areas all bikes. One time I cycled the camino was with a friend who took a Dawes Galaxy tourer - he mixed road and trail ad hoc without problem and thrived. On my, pre-suspension, mountain bike with panniers the walking route was fine. Another time with my wife we again mixed road and trail and added many ex cursis days visiting places without problems. Just check maps and avoid major roads and the plague that is traffic. The arcen, hard-shoulder, alongside major roads is often strewn with stones, glass and sundry flotsam wherein lies the puncture fairy waiting to bite the tyre of all that trespass onto her prickly domain.
Personally, I hold that nothing is lost instead a differing experience is gained by eschewing the beaten path of the multitudes in making one's own way along the route so I do not fret overly at stepping aside from the mandated route and finding quite and solitude just away from the path.