For example, are some settlers better at farming than others? Are some of them better at fighting? Or can everyone do every job equally well?
Settlers are either assigned or not. There are no statistics regarding settlers and opening up VATS on them doesn't reveal any additional information either. Even if they're not they'll still fight if an enemy approaches them.
The exception being that the named settlers from Sanctuary cannot be used to set up Supply Routes.
All settlers are non-personal which means that they can use any job you will give them.
There are no specials, advantages etc. if we are talking about simple settlers, not companions you sent to the settlement.
So if a settler doesn't do the job well, it means that there is no resource. For example, each settler generates 6 food, but if it is less - there are just no plants to use.