I've beaten every route on Easy, Normal, and all routes on hard expect B-1, and Extra.
I personally like this game because it's shorter than other games in the series.
I did a research on the freezing bonus, and it comes out as a not-so-simple formula.
Simply put,
00% - 0
10% -> 10000
20% -> 40000
40% -> 160000
60% -> 360000
100% -> 1000000
It is linear between the mentioned percentages.
As far as I can see, there aren't any crippling major bugs in the game.
The 'Give up and Retry' bug only seems to happen when the player chooses the second route option, then restarts.
Maybe it has something to do with assigning routes to numbers, then increasing the route number when the player selects the second option. It explains the A->B, B->C, C->Extra
It happened to me twice, once on Easy, once on Normal. When I was playing Normal B-2-2 after a restart, I noticed that the high score displayed wasn't the high score on B-2. After I beat the game and saw the endings and the stuff, I realized what was going on. Now I make a habit of hitting 'Return to Title' whenever I want to restart.
The bomb/snowflake count on the bottom left is messed up when the player uses a deathbomb.
EDIT : I noticed that Cirno dosen't die when colliding with the stage faries. Quite useful.