WP was made for the masses, this makes it what it is and generates differences in opinion depending on your experience and objectives. I write my own CMS but if I had to choose from other options I would go first with Drupal.
- WP is slow
- Security is an issue (yes, still is an issue)
- Security can be compromised by WP itself and also by the template/theme.
- Your customizations are not guaranteed to be centraliced, some might go on the theme and some on the CMS
- Upgrading WP have caused the death of many sites, emmm blogs.
- Automatic one click upgrades have done the same
- The same for upgrading themes...
- What took months of work could fall down due to incompatibilities among WP and its plugings after an upgrade
- The editor is terrible and makes changes you don't need/want
- The editor is too heavy
Very often, the owners of WP sites have to choose from upgrading and remove a plugin due to incompatibility or not upgrading and stay in risk for security issues. Also, very often WP owners face the "WP is getting too slow again" and no clue on why.