STEP is better though, especially when checking connector matings between boards and the odd components like switchs, LEDs etc. where it's better to have a more accurate 3D shape. FreeCAD will open STEP, but not IDF.
I get no issues with an IDF v3.0 output from Pulsonix 8.5 into DesignSpack Mechanical 2.
If you're using the latest versions of both programs then I'd start looking for something unusual in the design such as an odd board profile or cut-outs, or something a bit unusual with the component heights.
I know that the very first version of DesignSpack Mechanical did originally have an issue importing IDF which I reported to RS and Pulsonix but that issue was fixed.