if they are in reasonable shape they can give good service, but if they need parts it can soon turn into a problem.
In the rear hub, all the parts are 'special' to this setup except
a) the axle
b) the LH cone and both locknuts
c) some of the sprockets on the freewheel.
The RH hub bearing has a smaller than usual bearing cup and uses smaller than usual balls; wear here is common. I keep meaning to check if the cup is the same size as is found in a Maillard front hub; if so it could be replaced when it wears I suppose.
The freewheel bodies are the usual Maillard quality, i.e. dodgy tolerances but hard-wearing steel, and (certainly whenever I get hold of them) always need to be re-shimmed. They usually come good once they have been re-shimmed.
To remove the freewheel you need the 'beer bottle opener' tool that Maillard made for the purpose; you can easily damage the lockring by using other tools (eg grips) on it.
Overall I'd think twice about building a new wheel around one of these hubs, unless originality of equipment on the bike is a real priority.
cheers