association database

The Association that I work for is currently deciding between adopting an out-of-the-box membership system like iMIS, Avectra, etc or building a custom Drupal site and backend database.

It seems like going the custom route could potentially be a better fit but it doesn't seem like very many Associations are going this route - is there a reason that they're staying away from this option? Having to customize something from the ground up does seem a little daunting.