Just taken a folding wheelchair on a tender in Bantry Bay Southern Ireland.
I can manage the 16 steps as long as there is a bannister, which there is. My partner carried the wheelchair. All quite easy, crew are very helpful getting across the gap.
WARNING. Would not attempt it in rough weather and keep an eye on chair or rollator when boarding. It is left on the quay until all passengers have boarded, we needed to shout out ‘don’t forget the chair’
It is at the discretion of the crew, some landing places are unsuitable if you have mobility issues and some ports use their own boats as tender and will not take walking aids, e.g. Scilly Isles.