it isn't that thunderlords is op
Because its not. Its just that the rest of them are so underwhelming.
The only other good ones are the crit heals one, on certain champs who crit is an option, or the heal ever 4 seconds on attack, only on champs who harass with auto's, top champs mainly ranged like gnar or jayce, and only in lanes you feel you will need that extra sustain.
The rest? A speed boost when doing a big chunk of damage in a burst? Who the hell would want that. Needing a speed boost is so situational, and doing that chunk of damage is so more infrequent then the other ones I listed which are always viable.
The deal random damage over time? Dot always feels weak unless you are brand, and doesn't feel satisfying to use, you can't see it and ratios are kind of insignificant.
Its not that thunderlords is overpowered, cause its not, its just that most of the rest of them feel like a crappy passive on an item no one buys.