As a firm believer of squeezing the juice out of technology until the pips squeak.
These are some limited of Excel Services.
Features that Prevent Excel Files from Loading
In some cases, when Excel Services does not support an Excel 2007 feature, Excel Services will not load the file. For example, Excel Services does not support running Microsoft Visual Basic for Applications (VBA), so it will not load files with VBA in them.
Features that Are Not Displayed
In some cases, Excel Services does not display an object in a workbook that has full fidelity with Excel 2007. For example, charts in have new visual effects, but those effects are not visible when viewed using Excel Web Access.
Features with Limited Interactive Support
Not all features that you can interact with using are fully supported in Excel Services. For example, users cannot add or rearrange fields in PivotTable reports when working with workbooks in a browser.
You cannot author an Excel workbook using Excel Services. This means, for example, that you cannot insert a chart or change a formula once the workbook is opened in Excel Web Access.
However, it’s still a great technology to use with SharePoint and Office