Wow, that's terrible! Sorry you had such an awful CM interaction, they should be welcoming you and be encouraging - not rude and snippy, telling you to go elsewhere! Obviously a very unhappy and defensive CM! First time they've heard something bad about MB/FP+? Ridiculous!
I have to say I'm really disappointed with Disney throughout this whole business. How sad is it that things with all of the MB/FP+ switchover is going so poorly? So many CM's lack basic knowledge of the MB/FP+, cutting back training when they've spent so much $ on this new system already sets the whole thing back because guests can't count on and depend on the help from CM's to be accurate by any stretch. They should have spent w.e. extra is needed to get the CM's in various positions to know the info backwards and forwards. Instead they leave guests to practically fend for themselves. No wonder the CM's act poorly, they must be frustrated beyond belief, and embarrassed, too. I know I would be if I were working there and left looking like an idiot some days. That's pretty bad when people know they shouldn't trust what a CM says. (I realize some guests, many, have no idea, and blithely trust). CM's are the front line representing Disney to their guests but sometimes these days they come across as doing a very poor and embarrassing job, IMO. Makes me embarrased for Disney. The whole mess overshadows those CM's who may still be shining, I'm sure some still are. But, it's hard to see the shiny amid all chaos and mess that is going on.
Disney was like my hero, everything Disney was always wonderful, and I could always count on magic, now that's just maybe yes, but maybe not so much. And not being able to trust Disney and what they are putting out for the public? Very disheartening! They've got CM's lying to the public, and yes I believe that - many are untrained but I think some, like the OP's CM, I think she was lying pretending they've never heard any bad comments.

Another example would be the whole mess at Pop. It's tough being disappointed with your hero, and it hurts being lied to by your hero. And no negative comments will hurt Disney's reputation more than their own very poor handling of this huge new program they are forcing on guests!
I do have a trip coming up in 11 days and I am still very excited about our Christmas trip! Really!

But, there is a little bit of sadness I feel about how Disney is dealing with all of this. I thought they were an honest and upstanding business and now I've got much doubt about that. And I definitely don't feel as valued by them as I used to. I expected much better from them. Seems like they've lost all the Walt part of Disney these days.