It returned a value, but not the right one?

That's odd. There are some differences in functions but ISERROR is a pretty simple one.

I've had some errors after conversion but I have generally been able to fix them.

Yes, it returned a value but not the right one. The scary part was, I was about to hand over a workbook designed for specific calculations based on weights (weights of animals, in order to determine medication dosing). If I had not scrutinized/compared the same file on both programs, my friend would have gone on to use the workbook in Google Sheets, and her calculations would have been very very off, and animals might have suffered.