Hello Rubén, I don't see any disadvantages. In fact, I see only advantages using such statements. First of all, by including such statement, all authors commit to the fact that they are all read and agreed on the final version and they have contributed intellectually. Historically, in many countries and many academic centers, authorship are given to the chairman or Head of the department, even if no intellectual contribution came from them. Although that is not things suppose to happen, traditionally this has happened. Including such statement will revisit and remind who has actually contributed to gain an authorship and who can be acknowledged in the paper.

Additionally, in many plagiarism cases, the senior author will find an excuse that I didn't realize my post-doc or junior author might have done this. I was unaware of it and I want to withdraw my name from the paper etc. if any problem arises after the paper is published. Now this puts a barrier to such thoughts. The fact that they sign, by default, they are also responsible. So, they will reluctant to be an author if they didn't contribute or read the manuscripts carefully.

I think a author contribution statement is a good idea, and especially if it specifies the individual authors contributions. This will make the scientific process more transparent and I see no disadvantages for science except that contributors have to discuss the issue. This may not be so easy if the communication in the group is not open or if some contributors feel they should have credit exceeding their actual contribution.