I spent 10 days road riding on Ibiza last August, its lovely – got my hire bike from Kandani.es – they deliver round the island, was an Orbea Avant with 105 in really good condition, they also did Giant and had MTB’s from hardtails up to high-end full-suss. Not sure about MTB but certainly lots of quiet roads with little low speed traffic and I did see a fair few MTB’s on the back of cars – there are camino roads that lead into the interior.
Up near Portinax is the hillier bit but nothing is too extreme, we did the ’round the island’ ride which was fun although traffic in Ibiza Old Town and Sant Antoni was a little slow, also did lots of 50-100k road loops around the interior.
Veloclub Ibiza are worth a look on strava – they do road guiding, or as mentioned above Ride Ibiza