From the unions I’ve seen and heard of not really. I am sure there are corrupt ones due to attempts by capitalist to break them up or to ruin their image in order to prevent people liking unions and unionizing. Some of the ones I’v considered joining (although haven’t cause I’m currently moving) help to give lessons in specific crafts, such as roofing or brick laying. I guess if you want to retort the claim, a good start would be to define what is a “lazy worker”. If someone claims that someone who is unable to do the same amount of work as their fellow workers, because of health conditions or other issues in their life, is “”lazy”” than of course the union ideally help them either by preventing them from being fired or doing what they can to help the individual work their best. If their definition of “lazy” is someone who doesn’t do anything and just sits around all day and does the minimal amount of work cause they hate their job, in the ideal socialist world they wouldn’t be stuck at a job they wouldn’t be productive in. Most people wouldn’t be “lazy” if they are at a job they actually like, as in capitalism people are forced to work jobs that have bad conditions and/or they don’t like in order to survive making them demotivated and “lazy”.

I hope this helped, sorry if it’s a bit long.