121 is about procedures, protocol and profiles. They want you to do things EXACTLY as they tell you to. Flying the plane is almost secondary to all of that. I've been in classes with guys who were great sticks but tripped all over the airline profiles and checklist protocol (flows, callouts, etc.). In the end, that is what got them in trouble more than anything else. Think of 121 training like practicing an acting script - say and do everything exactly at the right time AND fly the plane as the COMPANY wants you to fly and you'll pass. Top gun it and you're done. Simple.

135 training usually takes place in a more "forgiving" environment such as CAE Simuflite or FlightSafety, where there is definitely an interest in seeing you pass (since YOU are the client and YOU [by proxy of your company] are paying for the training). They're not so much interested in how well you can run the prescribed script, but in your decision making skills and how well you fly the plane. So in that respect, yes that would be a challenge for someone who doesn't have the experience with decision making under stressful conditions.