There are three main things they did wrong:
1. Bad use of Epic's tilesets
2. Mix up the whole story
3. Game is too short!!!

Lemme talk some about the third one. In JJA there are 5 planets (Chameleon World, Space City Carrotopolis, Icebox3, Industrial Wasteland, Chelonia IV), each has 3 levels and boss level. That gives us 5x4 = 20 levels. And the levels are quite short, especially the ones on Chameleon World and Chelonia IV. You will probably say that you have seen more tilesets on screens. Well, there are 2 levels where you can die from falling out of level: Space City Carrotopolis level 2 and Chelonia IV level 2. First one uses sever part from Colon 1 tileset with added goo, tech-ceiling, changed colors in walls and removed "Ye Sever" texts. The second one uses tileset which I call "(Darn)ed Castle". It's simply Castle 1 + (Darn) 1 + fire from Castle 2. It is used by whole Chelonia, but has two backgrounds (cosmic sky and hell).

Derby: Filter bypass replacement. The rule applies uniformly.