The problem is not the CMS. I use Drupal for almost all of my sites and it's excellent, but if you don't create your own themes or modules you are very much in the hands of those that do.
Some themes are excellently written with SEO in mind, as are some modules, but others are slow and slow your site down. Also, I'm surprised how many themes are not geared for mobiles and tablets.
The best site in the world is doing poor SEO if it is not also reaching out to those accessing it via a phone. The best themes have one layout for widescreen desktops, one for tablets, and a further setting for mobiles, and the change is automatic.