The first one is about how well it is integrated in the show, or how "respectfully" it is done.
Let's say for example... Car brands are very welcome and a must in the Fast and Furious movies. It would be pretty silly if they had fake car brands in such a movie, and the presence of real car models can excite and make the enthousiasts nerd out more about them. Overall a victory for everyone involved.
But hearing how great of a family car a Volvo can be, in a movie about aliens... not so much.
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The second is about transparency. Assuming you do not like let's say Monsanto, or Scientology and you would like to avoid works that are funded by them. Product placement is a form of guerilla marketing, noone says who they took money from. So it lacks transparency and is disgenius to the consumers.
It's a bit like influencer marketing, when influencers "forget" to include the #sponsored hashtag.
User ksdale has left a very nice comment on the subject.