Important point, but from my understanding, EUPL code stays EUPL licensed code. So the original author has the right to use the modified EUPL code from a project which is GPL licensed as a whole, but uses EUPL licensed code.
I would like to have confirmation about this.
A disadvantage of EUPL vs GPL I see, is that if you want to include GPL code into your EUPL project, you need to relicense the project as a whole (not the UEPL part I think) to GPL.
A other disadvantage could be that EUPL isn’t compatible with future GPL versions (GPL+ / and later versions).
edit: compatbility is not reciprocal …
Yes, it is. The EUPL has a compatibility list, covering 10 other licences. In most cases, compatibility is reciprocal. However in the specific cases of the GPL and AGPL, compatibility is not reciprocal: for example, the EUPL is compatible with the GPLv3, but the GPLv3 is not compatible with the EUPL, because it has no compatibility list. If you merge GPLv3 code with EUPL code, the result become covered by the GPLv3.
EUPL - the free/open source software licence of the European Union
Maybe I’m confused with the MPL license:
In this case, the program as a whole will be under the chosen GNU license, but the MPL-covered files will be dual-licensed, so that recipients can choose to distribute them under that GNU License or the MPL.
Mozilla Public License - Wikipedia
That’s maybe something that they need to add to the EUPL license.