The difficulty with a number of CPC games was that very little time was given to do a port, so it was done the easy way. ie: Use mode 1 (320x200x4), shrink the screen to match the Spectrum (256x192), and make a more or less colourless version with little code change. The Spectrum was king in the UK, so maybe that some sort of weird, crazy sense. Ubisoft did it the right way, and ported CPC games from the Atari ST.
But there are plenty of examples of good games, too. Spent hours playing Target: Renegade back in the day... And the original Renegade was nice, too (and had better graphics than the other 8-bits

) Arkanoid, Rainbow Islands, Gryzor, Pang, among many others were all fairly well ported.