I believe the issue with the Data Viewer was fixed in V2 (that was beyond awful)... But there is still plenty to be addressed, particularly with the layout tools. While I have not yet personally encountered the Window bug... I have found FM12 more likely to crash than any prior version. I have mostly been running on the Mac with FM12. To be fair I am also pushing the layout design quite a bit further than I have in prior versions. Most of the crashing has occured in Layout mode.
Using the data viewer did improve slightly with 12v2 for me, but it is still essentially unusable when connected to a hosted file over the WAN. I find the performance of 12 itself over the WAN to be much improved from FMP11, but using the data viewer is still considerably slower. This is on Mac or Windows for me. I should qualify this a bit by saying that if the table(s) of the solution are small in the number of fields they contain, it does improve. When I say small though, I am talking about 10-15 fields. Once I have a table or two with more than this, it slows down quite a bit.
I'd like to add another bug that's left over from at least FMP 11, and is still present in 12:
In the "Save a copy as" script step, checking the "After saving, automatically open file" check box has no effect, and the new copy is not opened automatically. If one does this action manually from the file menu/Save a Copy As . . ." command, it works properly. Please fix the script command!!