If you’re going through HR, long cover letters are useless. If you’re applying directly to a hiring manager, they can be very useful.
I have occasionally had a sheet consisting only of bullet points about what’s great about me. Ironically, this seemed to resonate with HR better than a cover letter. In fact, one time an HR person called and said that they didn’t have my resume, only my bullet points, and they wanted to talk to me.
So, just remember that HR people spend their day scanning dozens if not hundreds of resumes, and anything that can make your point quickly and clearly will pique their interest (assuming that you are at least reasonably qualified for the position).