Excel was doing bad things to my data. Gramps names & places can include quotes & commas. Either the Excel export was not adding the right Escape codes or the Gramps import wasn’t parsing it correctly. Ether way, CSV was not working well for me.
@SNoiraud just did enhancements for Tab Separated Values instead using Commas. (Thanks Serge !)
The enhancements are targeted for release with the 5.2 version. But you may want to try patching a 5.1.x version.