Upon further reflection, I'd like to submit a name change to Marvel for the character, from Onslaught to Yawnslot. First syllable is for obvious reasons. Second syllable because it rhymes but is not quite the same, and does not invoke fear/trepidation/etc. within the reader. Heck, let's call him Yawnsalot!
Original join date: sometime in 2002
IIRC, the Celestials viewed Franklin's power as potentially being on par with their own. The Stranger and Dark Phoenix are the only ones on the list who might be able to hold their own against him.
Onslaught was an interesting idea that ended up suffering from the 90's "Extreme" fad.
I believe Phoenix, Dark Phoenix and Stranger to all be more powerful than Onslaught, at least the Onslaught powers that were shown.
Onslaught was the comic that made me stop reading superheroes.