[austincheney]

jQuery is constantly tested with all of its supported browsers via unit tests. However, a web page using jQuery may not work in the same set of browsers if its own code takes advantage of (or falls prey to) browser-specific behaviors.

https://jquery.com/browser-support/

That quote says that if you attempt to solve for a cross browser conflict using a standards based approach it will likely conflict with jQuery code in the page. Either your approach will fail due to changes imposed by jQuery or your approach will break something supplied by jQuery. That is bad mojo.

Another problem I have with jQuery is that it encourages extremely inefficient practices in exchange for convenience. For example consider jQuery's closest method, which looks great if you don't consider the steps that execute under the hood to make that happen. In contrast using a non-jQuery approach generally meant event handlers that more directly target specific nodes in the page without all the necessary DOM walking.

I also remember jQuery frequently breaking IE back in the day. IE uses instruction count to determine if there is too much code in the page, whereas other browsers use a 20 second clock to warn on long executing code. Since jQuery does a bunch of unnecessary things and encourages method chaining those instructions counts would quickly add up and IE would stop executing code.

I have also found for many developers that jQuery is/was more of a live or die crutch they cannot live without opposed to a time saving convenience which was very off putting because developers were unreliable when things broke or when things needed to execute faster.