This is a free software license. By itself, it has a copyleft comparable to the GPL’s, and incompatible with it. However, it gives recipients ways to relicense the work under the terms of other selected licenses, and some of those—the Eclipse Public License and the Common Public License in particular—only provide a weaker copyleft. Thus, developers can’t rely on this license to provide a strong copyleft.
The EUPL allows relicensing to GPLv2, because that is listed as one of the alternative licenses that users may convert to. It also, indirectly, allows relicensing to GPL version 3, because there is a way to relicense to the CeCILL v2, and the CeCILL v2 gives a way to relicense to any version of the GNU GPL.
To do this two-step relicensing, you need to first write a piece of code which you can license under the CeCILL v2, or find a suitable module already available that way, and add it to the program. Adding that code to the EUPL-covered program provides grounds to relicense it to the CeCILL v2. Then you need to write a piece of code which you can license under the GPLv3-or-later, or find a suitable module already available that way, and add it to the program. Adding that code to the CeCILL-covered program provides grounds to relicense it to GPLv3-or-later.
The fact that re-licensing from EUPL to GPL is so cumbersome (and therefor off-putting to independent developers), and that at the same time it allows for re-licensing to weaker copyleft (i.e. for derivative works to be more proprietary, so to speak), makes me not want to use it.
The Free Software Foundation writes the following about the EUPL 1.2 (https://www.gnu.org/licenses/license-list.html#EUPL12):
The fact that re-licensing from EUPL to GPL is so cumbersome (and therefor off-putting to independent developers), and that at the same time it allows for re-licensing to weaker copyleft (i.e. for derivative works to be more proprietary, so to speak), makes me not want to use it.