It never FEELS like it's that effective when you're watching enemy health bars, but in reality, it's doing more than you think. Do the math. At most 100 bonus damage to one target at the beginning of a fight from Deadman's, and likely not any afterwards vs. 25-43 every second to every nearby target for the duration of a fight from Sunfire? I think I'll take sunfire, thank you.
I've noticed in many games that a tank rushes sunfire, that tank tends to be the highest damage-to-champ dealer in postgame statistics, even if they went primarily full tank besides. There's a reason why over time Riot has made sunfire more expensive and less effective early game.
I remember a few years ago when I could rush Sunfire in 3s on Sej, forget everything else, including boots, and immediately start roflstomping opponents the moment it was finished. I would literally win EVERY 3s game that way. I had match histories that were just strings of uninterrupted Sej victories. And this is the pre-rework Sej. Ya know, the Sej that nobody played, and that everyone claimed sucked.
I may miss those days, but as a player who's not that skilled, who could pull that off, I can definitely say, sunfire was clearly broken when utilized to its full potential. It's fine where it is now.