Any Garmin GPSrs DON'T have custom TYP support

[Seldom]

I'm just starting to make custom TYPs.  I was looking at the MapWel site for a TYP editor, but they mentioned that some Garmin's don't support custom TYPs.  If I want to create a map for general distribution, will custom TYPs reduce the usefulness of the map significantly?

[maps4gps]

In general, this has been mentioned, however I have never been able to get any details on which units do not support custon type files, nor what effect it has (as in simply does not display the features, makes the mapset unviewable, or locks the GPSr).