1. Financial reports (General Ledger, Cash Flow, Profit and Loss, etc...) that US businesses rely on, leave a lot to be desired and/or need a lot of custom modifications to work. These things need to work perfectly in an ERP system, and sometimes the response from OpenERP SA seemed lackadaisical as if partially correct reports for the US companies would be OK.[1]
2. Things like reconciling bank accounts and taxes are geared towards a European system that seems much different than ours, and as a result cause a lot of problems in a US implementations. Again, OpenERP doesn't have a localized US package that addresses these issues in a concrete way. Searching for help, you'll probably be referred to half finished module, with 0 documentation that still requires a lot of work to be ready for production.
3. On the stock and manufacturing side of things the system works much better in my opinion. And with the right amount of work done up front, the they both offer flexibility while following a strict business logic[2] set in place at the time of implementation, that they can do some amazing things for a sales / fulfillment side of the organization. The reporting and automation that OpenERP can provide on this side is really awesome.
[1]. I am not an accountant.
[2]. It is a ton of work defining a business's process in the terms of an ERP. Sometimes I felt like OpenERP got blamed for a company poorly defining their process (if they even had one) So be wary of that when looking at forums and things...
Concerning financial reports, OpenERP 6.1 introduced the ability to modify financial reports (hierarchy, order, content, sign,...) directly from the configuration interface namely in Accounting > Configuration > Financial accounting > Financial reports. No need for custom development to get the reports that US businesses use.
