Can I use free asset for Unity in a non-commercial project?
That would depend on the asset in question and it’s attached license terms.
You have always to check the license terms but the generally answer is yes.
Most of unity free assets are suposed to be used in unity itself (like unreal does with its free assets), no matter if the project is commercial/non, check it, before u move any asset from engine to engine.
Free assets provided by Epic (ie: Paragon or Soul Cave/Soul City) are intended to be used only in Unreal Engine and it is forbidden to any other engine, under the risk of a lawsuit. It is clear in the public announcement and in the download page in the marketplace: “Licensed for use with the Unreal Engine only”.