All the gangs are viable.
Some are more complex than others. Some have far more options than others. Nomads are not terribly complex as a gang to build and understand, as they lack many of the options that have been added to other gangs. Their biggest challenge is that their play style is very much geared specifically to Ash Wastes scenarios rather than Sector Mechanicus or Zone Mortalis scenarios.
So it depends of the type of campaign you are joining.
Mounted models (half the gang) cannot see the table in anything but Ash Wastes scenarios. That includes their one fighter type who can cause the Visibility (X) rule to come into play if it isn’t already for a given scenario.
And so since that fighter is Mounted, cannot be fielded in most scenarios for non-Ash-Wastes campaigns.
The core rules for the Nomadd gang rather rely on the Visibility (X) rules.
So if you like them (and you should! Look at them!), talk with your playgroup about fielding that one fighter dismounted despite Riles As Written, or having some other way to cause Visibility (X) even without them on the table.
Cawdor on the other hand are a bit of a horde gang. You can almost always put a scattering of extra fighters on the table above and beyond the starting Crew size for any given scenario. They have a lot of access to Template weapons, so Ballistic Skill matters less, but they need to get relatively close to enemies to start being really effective, where the Nomads can hang back outside of visibility range and plink away with long guns.
Cawdor like the other five Hiuse gangs, have had a lot of options added through other books since House of Faith as well, so there is a lot you can expand into over time, later.
So the play styles of the two are very different. Both viable, but quite different.
Does that help at all?