1) Cool down.
I don't know how developers call it, I will call it cool down.
Cool down - time between accepted hits to harts or blocking squares (ice and etc.). In general it means that candies with simultaneous explosion will do only 1 damage instead of many. (Colored big candy/black candy in colored dots + wrapped candy).
In rare cases (I hate this most): Assume you have heart and candy with stripes directed to the heart right next to it. If you do match-3 with this candy and it goes off, the heart will move only one step instead of two steps because of Cool down. Heart gets "damage" from matching-3 candies then it does cool down and hit from stripe is ignored.
2) (black candy in colored dots + black candy in colored dots) is less useful (except corner cases) than 2 x (black candy in colored dots + any other candy). Because it does only one hit to every cell on the map.
3) Does any one know how a fish (with no jam on it) pick a target among blocks? among hearts? Sometimes I fill like it takes most useless target which will ruin my next AWESOME move.
Please add more useful information.