There are some bugs when working with custom filters or large datasets. I've had issues where the numbers in scorecards and graphs didn't match and re-adding the data source wouldn't fix it. I've also gotten different numbers using the Google Analytics integration versus the analytics homepage. There were a few times where a mixed data source would display no data even though it should have existed. I still like the software but I've had to double-check almost all of my data when I use the dashboard.
It also doesn't make much sense that the graphs in Datastudio are more limited than in Google Sheets. In spreadsheets, the software will interpret and graph the data correctly but if I try to replicate the result in datastudio, the graphs will sometimes break or display no data.
On occasion, the dashboard has trouble loading data at all but this is more of a minor annoyance since it usually fixes itself once I refresh. Review collected by and hosted on G2.com.