Don’t think the claim against them matters so long as they don’t have to many claims. I had 3 claims 1 for each of my first 3 vehicles on motability and it didn’t effect me in anyway.
As mentioned above, if you always intend to remain in the scheme it’s fine. The problem occurs if you leave, as you then start with zero no claims discount, whereas with a no claims history earned within Motability you can transfer at least some of it over to private insurance.