That is merely anecdotal evidence. Last I checked 195s and 170s were less reliable than the 15x as a whole. As
@Bletchleyite says, that sounds exactly like wheel slip and would affect any unit - not a unit fault.
15x only require simple conversion courses between the types, are well known and understood by crew and fitters, have a large overlapping spares pool and when they do fail can often be fixed by a man with a hammer and bag of spare parts. They and 170s are compatible in multiple so can also easily rescue a failed unit.
A microfleet of 185s (unsuitable and shoehorned onto another route) would be a nightmare in terms of planning, crew training, repair/servicing and rescue. A small number of unique units within a business does not make for a reliable fleet.