I think the biggest difference is that lifegain is already a useful thing for many decks (mostly slower ones), where being a Goblin is not a useful trait without any payoffs. On the other hand you make lifegain cards even more flexible, since even aggressive lifegain related decks will want them.
Mire Triton and
Faith's Fetters (and dozens like them) are very versatile cards anyway.
The only thing that is a bit narrow are the payoffs. But there are cards like these:
These get better with lifegain without requiring it to be good cards. Also, by now we've gotten a few self enabling payoffs at various power levels, probably more than most tribes have:
Also, most of these payoffs care about you gaining life, which can be achieved every turn with a single
Healer's Hawk in play. Most Goblin decks for example want you to have multiples to be really worth it.
EDIT: Oh, and I think it is easier to integrate lifegain into other decks, since lifegain is alreadya reward for doing something, like sacrificing (
Zulaport Cutthroat), putting lands into play (
Grazing Gladehart) etc. For example, I've played
Oathsworn Vampire in rakdos sacrifice decks with just Zulaport Cutthroat and a
Mire Triton and it worked very well.