I'm on a 90mg every 8 week schedule with Stelara, and I don't feel particularly rundown ay any point, but I do frequently have increasing symptoms (mild lower right quadrant abdominal pain) starting at about the 6 week mark. It goes away after I get the new dose.
As for antibodies, Stelara appears to be a lot less immungenic than the older biologics such as Remicade and Humira. Antibodies can still happen, but they are less likely to be the reason for any declining effectiveness of the drug.