A 147 at snooker is red, black, red, black, etc., and then the rest of the colored balls? I'd say that's pretty hard. Probably harder than a 300. Allison Fisher might disagree, but who knows.
The frequency of a hole-in-one is probably less often compared to bowling, but even terrible golfers can get a hole-in-one. Terrible bowlers aren't going to throw 12 strikes in a row.
Tim