Graphite moderated reactors are broadly fine, the problem was with some technical specifics of that specific reactor design, and the operational culture that surrounded it. After Chernobyl, those flaws were corrected and operation of other RBMK reactors has continued to this very day, with no repeats.
Graphite reactors are not a good idea because they inherently have, well, graphite. That can burn.
The worst possible case for water-moderated reactors is uncontained meltdown. And it's not _terribly_ horrible. You will get contamination with volatiles, mainly cesium. But there's not a lot of it in the reactor, so it'll affect only a small area around the plant. Some fuel might get initially mobilized by steam explosions, but again, only a fraction.
The worst case for a graphite reactor is an uncontained core fire. That can burn for weeks and spread a significant part of the fuel as particulates over large territories (Chernobyl).
Is it likely? Nope. But there are black swan events: earthquakes, mega-hurricanes, meteorite strikes, Godzilla attacks.
