Ages ago now, but I went with my mother on a Shearings coach holiday, something we'd never have considered previously, but she'd been let down on a non coach holiday to same area (Shetlands) by a friend.
Have to say I was incredibly impressed, despite misgivings! So unbelievably organised, everything taken care of - your suitcases taken to rooms and brought down etc.
Quite a few of the others never got out of the coach unless there was something to eat/drink - whether lack of mobility or inertia I don't know.
From what I saw - if it's still the same - I would think it pretty ideal for anyone elderly with poor mobility - as long as you could request suitable hotel rooms - walk in shower, no stairs or a lift, etc.
We thoroughly enjoyed it and were very impressed. (BTW I don't work for Shearings or have shares in them!)