It would obviously be a more difficult upgrade as you would be operating at least half the time to airports and through airspaces with which you are less familiar. I seem to remember Virgin had an issue with Airbus co-pilots upgrading to 747 skipper, which may have been caused by other issues. But I think the point is that anything that increases training risk on first upgrade is best avoided. Lots of people got their first command on a mixed fleet type e.g. 757/767. But most airlines seem to want you to do at least a year short haul before going further afield.