My MacBook Pro 2014 has 16GB RAM, SSD, and i7 quad-core, so no issues with it and El Capitan. But I really didn't anticipate any as far as hardware. More importantly, I have not encountered any major software issues with older apps. I'm always concerned about that whether a new Mac OS X or a new Windows release. This MBP came with Mavericks installed.

My Mac Mini late 2012 with 16GB RAM, 5400 RPM HDD, and i5 dual-core has no issues with Yosemite or Windows 10 Home. I was afraid it might, but both work well on it with no sluggishness. The Mac Mini is the bottom of the line Mac. My Mini came with Mountain Lion installed.