The other two were already answered by anon, but I'd like to say more on this.
Burst-streak Eidolons are the second best Eidolons you can have. While they don't compare to P2W ones, Burst-streak ones give you a pretty huge boost to your bursting turns. However, notice that event Burst-streaks are just damn bad (Ixion and friends) due to their low Character Atk, the good ones are from premium gacha (Ifrit, Fenrir and Giri). The harder you hit, the more you gain from them.