The Atari ST is similar because it uses a 68000 clocked at the same speed and a bitmapped screen using bitplanes. The Amiga can be regarded as a superset of the ST.
The X68000 has lots of sprites, a tile mode for background graphics, a text mode and multiple bitmap playfields that can all be superimposed. It's not even a superset of an Amiga, it's just alien.
By comparison, the Archimedes is much more similar.
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