The conclusion of  A Few Reasons Not To Use osCommerce For Shopping Cart Software I will give three additional reasons why e-commerce store owners should stay clear of osCommerce. 

Password protection
One of the most frustrating parts to osCommerce is that there is no inbuilt ways to password protect the administration area.  While this can easily be corrected by finding information of password protection on the oCommerce site, it is frustrating that this does not come with the default installation.

Attribute system
If your business relies a great deal on attributes, osCommerce is probably not the cart system for you.  osCommerce has many limitations to their attribute system including there being no SKU identifiers and no stock control.  Some of the limitations can be modified and upgraded, but it is recommended that you select a different cart system if you depend on a quality reliable attribute system.

Large product catalogs
The last downfall worth mentioning of osCommerce is its limitations with large product catalogs.  When dealing with large product catalogs of over a hundred, modules for the input/management of data is required.  If the product catalog is even larger than a couple hundred, there are several other modifications that must be taken care of.

osCommerce is a decent open source based online shop, but there are several downfalls that should be considered prior to getting into it.  Ecommerce shopping requires a very reliable shopping cart system and open source based system.  While osCommerce has some great things to offer, there are many things that it lacks that other systems make up for.