Size is everything! If she says otherwise.. she's just protecting your ego : )
I think the 'it can be chump blocked forever' argument is dumb. If it kills something of your opponent's each turn.. that's a really good thing. You could make the same argument about Nemesis of Mortals, Arbor Colossus, etc. Just bc it doesn't have evasion or trample doesn't make it bad.. it makes it balanced.
I feel your comments about ETB, card advantage, etc. are true.
Haha!
*Insert "motion of the ocean" colloquialism here*
Anyways.
Obviously I didn't mean that it can be chump blocked FOREVER. You are correct that being able to attack in and kill an opponents creature each turn isn't at all a bad thing as well.
What I am saying though is that the current meta that exists just works against it so hard.
Most of good to decent Aggro decks don't care about you dropping
Fusion Elemental. They either have plenty of ways around it, or have you close enough to being dead by the time it comes down where it doesn't matter (at which point they can likely finish you with Burn/Evasion or by making bad attacks just to push damage through, which Fusion Elemental does nothing to prevent). The card often does nothing to help you come back when you are behind, unlike cards like Pelakka Wurm and Runescarred. Not to mention the power level and abundance of token strategies, which work great at chumping Fusion Elemental and/or going wide enough where it just doesn't matter if it hits the table or not.
Most of the good to decent Control decks are packing enough removal/bounce/discard/etc to likely answer it on sight and it has no form of protection to keep it from being vulnerable or ways to recoup the value lost to eating removal.
Most of the more combo oriented decks (most of the infinite turn decks, most Reanimator/Spawning decks, Seance decks, Lifegain decks, Jalira decks, etc) survive the early turns by flooding the board with stuff like Elvish Visionary, Satyr Wayfinder, Lone Missionary and the like, which people don't mind offering up to the chump block gods, and in a lot of cases, actually WANT to be chumping with to fuel their combos.
To be honest with you, I don't even like
Nemesis of Mortals, even with the exposure (and success) it gets from the frequent posters around here. Beyond that, it is overall a better card than
Fusion Elemental is anyways. True, both are undercosted fat, but keep in mind that
Nemesis of Mortals is much less taxing on your mana base, and has the potential to be better value in terms of economy (8/8 for 5 is good, but 5/5 for 2 is better, especially when it becomes a 10/10 the next turn for a potential investment of only 2 more mana). Nemesis actually has the support (from other cards) to consistently be cast for 2-3 mana, and while
Fusion Elemental may always cost 5 without any other fuss, it suffers from a very taxing mana cost (and the existence of only tap dual/tri-lands) which further muck with your ability to play it on curve.
As for
Arbor Colossus, that isn't a card I am too huge a fan of either. I won't lie and say I haven't included it in more than a few of my decks. That said, literally every time it was included it was done so specifically because the deck lacked any other viable (or significant) ways in which to deal with flying threats. In my experience the card doesn't get played because it is a relatively undercosted beater. It gets played because it is one of the very few things in the game that can profitably block most flyers and the Monstrosity ability is just gravy when many of the games good expensive bombs just so happen to have flying (Demons and Dragons and Angels oh my!)