Having run a bad D&D campaign before, you can bog the plot down in dumb ways, make poorly calibrated and repetitive fights (it is fucking hard to make good fights in D&D because the system is extremely obtuse and has no clear rules about target numbers and player stats to let you make appropriately scaled generic enemies flavored however is narratively appropriate, so you're left trying to mix and match a handful of narratively appropriate monsters from the big list of pregenerated monsters based on horribly inaccurate rules of thumb by party level), or get lost in fiddly little details nobody notices or pays attention to because there's a world of difference between setting the scene of a ttrpg and setting a scene in fiction writing.

There's also the option of running a pregen adventure, which will be even worse because those are awful and even harder to run than just winging it is.

Really D&D is just not a good system in general, being overcomplicated for how barebones it is and overdetailed for how generic it is. Does anyone need hundreds of pages of spell entries that are just different flavors of direct damage combat spell, tiers of the same utility spell, and a ton of super niche effects that no one will ever think about taking because they only get to know a handful of spells and have to prioritize taking damage spells to remain viable? No, it's fucking awful and even Pathfinder - which does it a bit better - still runs into that same core problem of "99% of this stuff is super niche, so I guess I'm just sticking with the straightforward damage and CC spells that'll actually get used." Does anyone need a thousand pages of shitty monsters that are fully statted out in ways that are 99% redundant, from which DMs are expected to build encounters where literally the only numbers that matter are health, its attacks, and its AC? Nope, not even a little.

I've also run the odd bad Shadowrun game, but that's a bit harder to mess up because of how much easier it is to stat out generic enemies with a basic understanding of dice pools (they just get 6-12 dice for a check as narratively appropriate and that can just be decided on the fly instead of written up, anything higher and they're probably something that needs an actual stat block). That I've messed up by fiddling with plot ideas too much and making a run end up nonsensical, or by letting combat bog down into a multi-week ordeal by tracking movement and exact positions - never ever do that in Shadowrun, make it fast and dirty so the combat's done in half an hour instead, and anything more involved should be handled narratively with generic skill checks not fiddly "ok you can move up to 100 feet this turn" shit.