Hm...As far as source go, when it comes to the OP in my own quick experiment they certainly roughly could almost match the volume of migration in two years or so, thanks to performance tropes (I also find those can have different composition from regular visitors).
But ignoring the large and potentially quite useful racial advantages, I've I think they tend to have worse useful skills on average - for me, I think the average visitor is up to 11 dancer/poet, 3 in weapon skill and 1 in discipline/armor user/dodger/shield user and usually also novice in 1-4 civilian skills - regarding the combat skill, I've never seen a visitor without at least that much, in fact.
Still, not exactly prime acceptance material. For the higher end, I do see visitors with one to several useful great skills, and occasional decorated/masterwork gear.
- Teacher is a common one, gained through having one or more apprentices. Not too useful at start with the typical lackluster combat skills, but it's slow to rise iirc.
- Speaking of apprentices, I find that those with civilian great skills tend to have been one at some point of another.
- One can have great in random civilian skill(s) on Bards, but you generally can train up to there in 2 years. I suppose animal trainer might be one notable exception, for instance.
- But I think Scholars might be most useful on this front, as their great skills are more likely to be in Studying and 1-5 medical labours. I think this might be the most humane way to get a good medic in under a decade, if worldgen smiles on you.
For that, I find that if I place a library some time after tavern, the scholars have quite a bit of trouble showing up. Combined with the container bug in 2nd designated zone in 43.03, it makes sense to designate library as the first location.
And regarding the above, what I find weird is that skills tend to be so often either novice or perhaps adequate, or most or all the way to great (scholars expected). Migrant skills tend to be more evenly divided among levels. Of course, you can keep visitors in a pond training swimming for stat bonuses for a long while, while migrants need to eat, drink and sleep. Then again, can still train most skill to great in 2 years and also max physical stats, though it'd be easier to just wait.