Instamine is not a scam of its own but it always brings in a risk. If you consider the fact that Satoshi Nakomoto is said to mined 1 million bitcoins and owns it, same logic means bitcoin is a instamine and premined scam.
I think the only possible worse case scenario on Dash premine is if the developers or whoever holds those premined coins starts selling the coins in order to take profit, if it stays like that there is no problem at all. This creates a doubtful cloud over the buyers but not a huge one.
I realize people on this forum generally aren't that bright so i'll try to explain it slowly without using too many big words
Dash:
8 hours to INSTAMINE 1.5 million dash, emission changed from 500 to 5 after the instamine
Bitcoin: took
about a year to MINE 1 million bitcoin, no emission change
GOT THAT?
If the dev lies and makes excuses like Evan Duffield did, trying to downplay the instamine and act like it was an accident (oops!!!) then yes, it's a scam