My OLD machine will sew through Leather that is 1/4" thick. Denim is nothing for it to sew through. Just remember to use the correct size needle.

I got a new Janome a few years ago - it did not sew through denim at all - I called the company and they told me I needed a heavy duty machine if I wanted to sew denim. That really bummed me out. The way the front of the machine was shaped I was having to strain my neck to see the needle. I got rid of the thing - vintage for me now.