Magento is a great product to do E-Commerce based on the PHP technology. It is a proven product with an open source version and one enterprise version.
The start with Magento is easy for the basic functionality that offers an E-Commerce Solution. The complexity can come once you want to implement custom functionality or custom behaviour and want to implement some modification.
Mangento is providing some extension on the marketplace that you can purchase. Here is the point the customization is not easy if you compare it with Ymple.
Ymple is easy to customize with modules and templates. The core structure is done in a way that you can modify the core to implement new functionality or use a new template.
An other point is that it is easier to build an E-Commerce mobile Application with Ymple due to the integrated API functionality. Ymple propose some open source project with the android / ios implementation that is a great feature if you want to do E-Commerce on Mobile and Web.
Some people prefer also to work on modern stack as Node.js instead of PHP.