The main problem with the 👎 reaction is that it does not carry any constructive feedback or explanation.
In some cases, it is added to a pull request early on, before the author has had a chance to improve their proposal. The person placing the reaction often will not receive a notification of the improvements and even if they did, they may forget to remove the 👎.
These effects are not obvious to the author or the person reacting; let alone to the reader. This causes friction for everyone involved.
If your argumentation has already been given by someone else, give their comment a 👍. That way, it's clear why you didn't like the pr at first and it allows the author to address the issue.
If the reactions stick around on a comment, that's not so bad, because it's quite clear that the issue in the comment may be resolved later on.