Being able to find a group with your undergeared alt isn't the topic here. The topic is whether certain specs will have a disadvantage when it comes to finding groups. The answer to that question is certainly going to be "yes". You will probably find groups that are willing to do your one keystone per week but that's it. If you want to actually do more than one set of mythic+ dungeons (however many you can get out of your keystone), people will avoid you if you're not playing one of the stronger specs out there.
If you're actually interested in doing mythic+ dungeons (and not just the weekly chest), you'll be at a severe disadvantage when it comes to finding groups if you don't play any of the specs that are perceived as strong.
- - - Updated - - -
Definitely outlaw. It scales much better with the artifact ability and artifact than assa so with the initial ~13 points it's already even with assa for single target and only becomes better with more artifact points. In addition to that, it actually provides the cleave you want for the medium to large pulls you often do with tank/heal CDs. Assa simply takes too long to ramp up if you multi-DoT with rupture and doesn't have the AoE for larger pulls either.
Sub currently seems sub-par because it offers lower single target DPS than outlaw at all gear levels available in the first tier while having weaker AoE as well.
If I had to put DPS specs across all classes into mythic+ tiers (1 being the strongest such as fire mage, 5 being the weakest such as frost mage), I'd probably put sub/assa in tier 4 and outlaw in tier 2.