Doctrine v doctrine is one thing but when you bring up flaws within their own doctrine without comparing or stating you believe in another doctrine plants a seed as @the girl next door says...and eventually that can lead to a dub asking questions about their own doctrine for if every dub, before they become a dub, reviews and researches the WT doctrine on its own, do you think they would still become dubs?
No.
Whilst you will never get a JW walking away by pointing out the errors in JW doctrines because they believe that the GB are in Jehoovers hands you are planting a doubt about that very point...and once they get to the point that they contemplate that the GB are NOT in Jehoovers hands or the society is NOT Jehoovers channel used on earth then the house of cards comes tumbling down fast but the starting process is very small.
An excellent way I use with my wife is to pull apart other religions doctrines the same way I would the JW's by saying the new light method doesn't work and can't be the truth or mention the child abuse and policies and whilst not referring to JW land the point is the penny might drop that JW doctrines fail like all religions doctrines fail.
Point is though, doctrine bashing will not result in a JW walking away...if done in a mild manner it can lay the groundwork for doubts.