As a human that still doesn't have final hit (I don't believe siraaron III or a Ibid have it yet either) I can say that humans are NOT shoehorned into final hit. Is it a huge advantage, yes. But humans have so many advantages it really doesn't hurt them that much. Giants and elves are fine as a concept, but their advantages really need to be exaggerated. Take away the extra AP costs (This is really the big thing), and maybe raise their bar and stat growths a bit more. Maybe give elves the ability to use 2handers/heavy armor, because their melee game, a key component to saving a character from a fuck up in archery or magic, is gimped to the point where it's painful to try, at least early on.
Giants DO make better fighters then humans, the chain mastery thing allows heavy armor much sooner then a human (it really does help keep you up in multi aggro which fighter gets a lot of so this is a pretty amazing advantage), and on top of it they have more will growth then humans, which effects knuckle damage. If you wanted to get nitpicky their hitbox is also larger so their windmill is slightly larger (it's a relative increase so it's not actually an advantage but shhh).
Throwing attack is HIGHLY underrated, rank 1 throwing attack does something like 900% damage on an already physical bruiser, twice the damage of a human smash, AT RANGE, it runs off strength, it has no aim time I don't think (although you can still miss, it's basically set to maximum aim depending on the circumstances, 90% if moving, 95 if standing still), and javelins have very high damage mods (as a note Hillwen engineering can make new javelins that are even better then long javelins! Although not metal javelins, which add a whopping 400 min and max damage.) which generally results in it basically being a huge long range cannon for giants that can only be fired once a minute or so. That's not a downside. That's a lateral upgrade.
Which really results in the issue, for giants, their "disadvantages" are mild issues. Javelins are huge and powerful but only work once in a while, making them impossible to use as a main skillset. There's still a bonus in there, I'd go so far as to say it's almost an advantage. Magic amounts to -2 int (literally nothing) and -24 mana(-1 int spell cast maybe. Minus several full bolt spells, but firebolt will still clear a room in a mana bar anyway.). Spells also cost a little more to cast, which any mage can tell you isn't a huge deal if you're stocked up on MP pots like any good mage is, although it isn't insignificant. Their stat growths are terrible for it, but besides mana, it's not a big deal because Int is pretty bad for damage for magic. Giants might have to avoid using mana shield, which isn't a big deal because they have the HP to take it.
Arguably the issue is that elves have disadvantages in the core skillset that some people would go so far as to say that everyone should max out before anything else, and giants have issues in the auxilary skillsets. -HP and -Str is just naturally a bigger penalty then -MP and -Int because strength is harder to get and when you run out of HP you die, as opposed to just being unable to shoot lightning. I would argue that's not entirely untrue, but I really feel that they were harder on elves with their disadvantages which is the bigger issue.
Of course, despite all this, merely due to the nature of mabinogi, if you want a melee elf, it's still more then possible. Your biggest issue is your strength, so solve that first. Carpentry, mining, combat mastery, you name it. All the gun skills give a decent amount of str too. Keep in mind the will you get will also raise crit, so it's hardly useless. You'll be fighting growths, which is a big deal, but it's still possible. You might have to focus more on damage enchants then crit enchants to make up for your bad strength growth, but some people do that with non elves anyway. You will never match a giant warrior of fairly equivalent strength in stats and enchants, but a giant mage will never match an elf mage under similar circumstances either.
This wall of text brought to you by me not using the quote button ever.