I have no familiarity with Nebiyu or Sjaak II. If all you do for these is specify a FEN, they are much less capable than Zillions-of-Games. Zillions-of-Games is very easy to program for new games, as the work of programming a game engine has already been done by the developers, and all that's left for someone wanting to program his own game is to specify the setup, how pieces move, and any additional rules. This is why we have close to 1000 Chess variants programmed for Zillions-of-Games here, and the Zillions-of-Games website has many more games we don't have here.
I have no familiarity with Nebiyu or Sjaak II. If all you do for these is specify a FEN, they are much less capable than Zillions-of-Games. Zillions-of-Games is very easy to program for new games, as the work of programming a game engine has already been done by the developers, and all that's left for someone wanting to program his own game is to specify the setup, how pieces move, and any additional rules. This is why we have close to 1000 Chess variants programmed for Zillions-of-Games here, and the Zillions-of-Games website has many more games we don't have here.