In 1984, how much do Winston's strengths and weaknesses lead to his success or failure?
There are many ways to look at it. The last part of that question is the first thing to consider: does he fail or does he succeed? Further, what is he attempting to do in the first place, where success or failure can be debated?
To address the first part of the question, I'm not entirely sure that Winston has many strengths. If you think that he has failed at his goal, whatever that may be, his weaknesses are certainly part of the reason why.