I don't have a problem with slideshows, but a reason is that, in the UX perspective, elements that animate recurringly with previously hidden content (and possibly links) is confusing and not actually used, and then when you find something you want to check you have to use the slideshow pagination, which is cumbersome. Better to properly display the points of interest.

I was talking to one of the Facebook cofounders years ago, and he put it better than I could (he wasn't talking about slideshows specifically). I don't have an exact quote but it went something like this:

We don't optimize for impressions per session. Instead we optimize for the number of things a user does in a fixed amount of time, (or per session). The former is a poor proxy for the latter, and you can increase impressions by adding useless click through pages that ultimately drive away users and destroy your business in the long term. (In the short term, these things make your metrics look good.)

I bring this up because slideshows are a great way to artificially boost impressions.

They are used by people who need to turn what should be 3 bullet points into a slow "presentation".

I don't like them because they make it difficult to copy and paste the whole of the content.