Success is having a goal and achieving it.
Not everyone wants the same thing - this took me a while to understand and longer to accept; why didn't everyone want to run/play the kind of game I did? Maybe if I only explained it clearly enough they'd agree? So I took a long time repeating myself in slightly different words until it dawned on me (I'm not the sharpest knife in the drawer) that different folks legitimately like different things.
So for example it's entirely fine to want a tiny very niche game that caters to your exact itch; maybe it's a book series you just read that's sorta kinda popular but not really, so you'll never get many players. How do you define success in that? Probably in getting the mechanics to work for its theme and getting the (probably few) players you get to buy in.
Other folks, like me, like big games - those are meant to attract the largest number of players, and a core idea behind it is to build RP momentum - if you have a large enough playlist, the idea is, you maximize the chances someone will want to play at the same time you do, and there'll always be things happening somewhere on the grid. What's success? Maybe getting all those people to not hate each other and keeping theme somewhat consistent since many of them will (surprise!) have a different idea of what the MU* is about.
Some people just like an extra polished game with a tight theme ran on a very specific vision - Eldritch is a good example of that. @Coin (<something about his mother>) ran it and it either worked for people or it didn't; it wasn't supposed to be everything for everyone. Others just want a sandbox and we've seen several of those - the game runs, it's there, +jobs eventually get done... and you can go do whatever you like. What's success there? Probably that it continues to run while there are players left with stories to tell. And some MU* are meant to be vanity projects ('I run this because I can'), often devolving into dictatorships or deserted glorified chat channels. How would you define success there? I wouldn't know.
My point is... each game that opens has different goals. It's very difficult sometimes to reconcile what the players and what the runners want because the former want something the latter never put on the table.
When picking a game actually figure out what it's offering as soon as you can then ask yourself if it's what you're after. Don't try to change it to be what you want unless invited to do exactly that or it will become an exercise in frustration for everyone involved pretty quickly.