Maybe so, however i was playing a game a little while ago and someone after gaining like a 6 kill streak gained an artifact and was unstoppable for the rest of the game
Hey - so we agree... probably best not to let anyone get artifacts. Put it this way - in my last 300 games, I have not had a single game where anyone was able to buy an artifact. So, for me, its a bit of a moot point. Perhaps you are playing games with high gold settings and this happens a bit. I don't generally do that. I just play on default settings.
But, let's talk high gold then. One demigod in particular, imo, has a huge advantage with artifacts at level 15 that no other dg could stand up to with similar items. That's ub. He gets acclimation at level 15. Anytime he gets hit for 500 or more damage, he receives 40% less damage for 5 seconds. Put that together with the bulwark of ages (which 100% of the time reduces damage by 25%) and you have a very hard to kill ub - that's going to be popping sigils anyway. And that's with just one artifact. Give him ashkandor while you are at it and he's a complete nightmare.
So, I'd say if we are comparing apples to apples (dgs at level 15 with the same artifacts), ub has an pretty big edge. A reg focused on auto attack can certainly do a ton of damage with ashkandor of course, but the ub I described with the same kit would tear him up.