Even though my experience is from a few years back, I absolutely cannot recommend Drupal for complex, custom applications. See my write-up here: https://github.com/rshekhtm/VentureTap/blob/master/README.md, especially the Lessons Learned section.
So much has changed from Drupal 5 that I don't think your experience is all that relevant when assessing Drupal anymore. Just as one example, Drupal 8 now does use object-oriented PHP extensively.