The only reason why Unreal and Unity are mostly mentioned around is because they gives a free prototype bundle (package) for Unity and, obviously, Unreal is popular.
The only thing you got to remember is that the assets you use should be made by you or on the public domain. This means that, for example, you cannot use the default map pack or characters or musics or sounds that comes with certain version of RPG Maker (this is just a good example I know). Certain engine includes the default files IN the project folder when a new project is created while other has it included in the engine's installation folder. As you got to share the source files (the in-engine folder that contain your project), those default assets might come with a redistribution restriction. If the default assets comes within the engine itself and don't have to be included with the project's source files, you can use them.
To put it in a simple way, if you can't use an asset that comes with an engine in another engine, you can't use it for this jam and got to make your own. That's usually a good way of identifying which assets can be used or not. If there's no restriction on the included assets and they only give it away as part of the public domain (only a few engine does this), then you can use those assets for this jam.
For example, Unity comes with default assets for primary models (cube, sphere, capsules, etc.) the UI (textures) and fonts for the in-game text. Those comes within Unity's installation and are not included in the project folder itself. You can use those. Certain version of RPG maker generate a list of sounds and musics (.wav and .midi files) IN the project folder every time you create a new project. Those CAN'T be used as they are restricted when it comes to redistribution. (It's in the legal wall of text you accept when you install the engine.)