[randomITperson]

Yes they are. I worked at a place where I had to build some web based reporting/BI apps around the data from an overpriced, proprietary ERP system designed for manufacturing. The Windows based client could have just as easily been a web interface and everything was stored in MSSQL DB's. Granted, there was a crap ton of tables, but nothing that complicated. Once I dug through it all, there really wasn't much to it. Most of the calculations it was doing were straight forward arithmetic. The complex part was just understanding the processes and workflows involved and how they related to the business side of things (ordering, manufacturing, invoicing, purchasing, inventory, accounting, shipping etc, etc.). But technically speaking, it's not that involved. My take away from this experience is, if you want to build a successful ERP app, it's all about usability.