For me the great handicap is the slowdown also. Some collisions could be better and in toy land is hard to get all the diamonds, and specially in order (can't recall all the details because the video was recorded in mid 2011 I think

).
The rest is great. Only has the 7 rainbow islands, there are no 3 extra island as the arcade version it seems. MSDOS version (I played the enhanced version as a kid), was similar, they didn't include those extra islands.
Secret rooms are there, but I supose to avoid extra memory problems they didn't add the door at the boss rooms, so they put the special item you could find inside instead. Shoe/perma speed in the first one, etc.
Also in arcade version there were codes like the ones Cesar included in Bubble Bobble for CPC in each of those secret rooms.
As they are not there, I supose they arent coded and included in the game. You could make those cheat codes in start menu of arcade to start with the effect of the room item.
This game with the smooth play of BB4CPC would be ubber and maybe the extra cheat codes / 3 islands would be wow

We wanted to record the 3 different ways to play the game from different users, and now jgonza recorded the last one we completed 'the project' and that's why I uploaded the video:
Bad Ending: (Recorded by jgonza)
Rainbow Islands - Amstrad CPC Longplay (bad ending) - YouTubeAll diamonds, no order: (Recorded by 6128)
Rainbow Islands AMSTRAD CPC Longplay ALL DIAMONDS - YouTubeAll diamonds in order: (Recorded by metr)
Rainbow Island Amstrad cpc HD + Habitaciones Secretas / Secret Rooms - YouTube
(don't remember the embed code)