I think the problem is less the size of the card pool (although it is a tad small), but the degree to which black control is simply massively overpowered.
Black has: Giant scorpion, vampire nighthawk, murder, assassinate, rats AND gravedigger all at common or uncommon (plus sign in blood to refill your hand). This gets even worse when you add in blue control elements (unsummon and essence scatter) or white removals (fiend hunter, o-ring and condemn) and it should be pretty clear why almost every single online deck is B, B/W or B/U (with U/R being the next most common).
Almost every pool will have strong black, but haste/hexproof boots or grave titan is in every highly-ranked deck (my pool is B/W control with grave titan and only loses to boots due to only one artifact removal). If you're lucky and get one of them, you will win a lot online. If you don't pull one of them, you will lose to decks that did.
So yeah. Sealed fun initially, but it's balanced terribly and every online match basically boils down to who gets their bomb card down first. I think if they actually let people reroll slots more aggressively, it would become a lot more obvious how homogenous it really is.