Should add that special weekend gachas and guaranteed 5*/6* pulls don't count, unless it's excessive even for those.
For example, there have been some gachas with an extreme increased chance to pull rainbows ( I believe it was 10 or 20 percent). Pulling 2 or 3 rainbows on those is fairly possible. Not super likely, but not super unlikely either. On the other hand, pulling 8 or 9 rainbows on such a gacha could be regarded as rather extreme.