It’s not necessarily about Motability.
My experience of Mercedes dealers is very variable and we have both our Motability car from Mercedes and our company car (a GLC premium plus) from Mercedes and both our previous cars were Mercedes. In some dealers the sales service has been shocking regardless of whether we are spending £40k+ on a car or getting a car through Motability.
Currently we use JCT600 in Doncaster and they have been really good (they have only supplied the Motability car so far). However prior to the latest car we used Listers and for many years they were great but then started to take our custom for granted so we moved dealers.