You do not know enough Mega Drive games
Many games (Sonic 3 for example) got much more on the screen than even an Amiga 1200 could handle.
If it would only be about colours, the Amiga would be better (the bigger palette (4096 vs 512) was clearly visible in many games, especially when the copper was used cleverly).
But if you use real Paralax-Scrolling on the Amiga, you have two layers with eight colours each or something like that. The Mega Drive beats that easily. You can see this clearly in Turrican 3 and you can see it not so clearly in Lionheart: The first world uses paralax-scrolling and everything, even the enemies, is green (using copper effects this green changes from the top to the bottom of the level). The second world is colourful, but it only has a copper-background.
The big reason, why Lionheart looks so much better than Turrican 3 is simple: Lionheart was designed for the Amiga, Turrican 3 / Mega Turrican was designed for the Mega Drive. The capabilities of both systems are still so close that a conversion always looks poorer, both have different strengths, which should ideally be considered in the design of the game.
Also the Mega Drive can draw many many more sprites. The explosions in Mega Turrican are much bigger than in Turrican 3.
@Keropi: No, the boss was added after the name was changed (it is supposed to be somebody of the Universal Soldier movie).
If you like Lionheart, have a look at Flink, done by the same team. The Mega Drive version looked better than even the CD32 version.