The differences are as you said. No Open-Source engine is capable of being a “Unity”.
jMonkey games Can look great, but its up to the developer to make them look great.
You can explore the Monthly WIPs, you’ll probably find something that looks really good.
This is a good one, Hoodwink by @Ecco.
Some pics:
Not sure if these are from the same game, but are still by Ecco.
jMonkey has tutorials for PBR, which is key to making “great-graphics”
https://jmonkeyengine.org/showcase/pbr_tutorial/