An important thing to consider when discussing Systems is where did it come from? What are it's limits? Is it a fundamental law of the universe, or was it created by someone? For what purpose? This can drastically change things, because if it's fundamental, then it might not be exploitable and there might be no theoretical limit to anyone's growth, but if it was made by someone, depending on who and why, it could have faults that could be exploited, inherent imbalances, limits. The System could be neutral, an ally, or an enemy, depending on why it was made. It could be omnipotent and omniscient, or it can be constrained.
And whenever you write a story with a system, you should first ask yourself if you can tell the story you want to tell without it. If the answer is yes, then you shouldn't bother with a system, it'll just bog things down with meaningless numbers that don't actually mean anything. If a story has a system, it has to be somehow relevant to the plot or essential to the world, either by allowing things that otherwise wouldn't be possible for the characters/world, by acting as a pseudo character in and of itself, by shaping civilizations in the world, or something else.
It's also notable that not every system has to have numbers, or even progression. A system could be as small as everyone gets one skill at adulthood, for example. There's no progression, but it could still have large implications in world building, with inherent inequalities based on what kind of skill you have, class systems based around that. Having a single skill might make characters need to be more creative about it. And it can serve a roll to give characters unique powers that would otherwise be difficult to fit into a setting or explain just how they work, or would be too difficult in practice for a person to actually use without 'system assistance'.
Personally, I prefer systems that are light on the numbers and numbers of skills, and focus more on the unique powers afforded by the skills, beyond a the simplistic stuff like 'oh, increase fire damage by 1% for each level in this skill'. That type of skill is just not narratively interesting, and it's hard to see the effects in the story. It might as well not be there.
Skills should have direct, visible effects in the story, and advancing the skill should have tangible, immediately obvious benefits. That is much more satisfying for a reader than just, 'number go up'. Eventually, number go up becomes boring and just gets glazed over. Skills having levels is okay, but I believe they should not be the sole thing going on. I believe that levels should act more as progress bars to the next milestone, which fundamentally alters or expands the skill in some way.
Skills are more crucial to a system than stats. It's the skills that make or break a system, by both meanings of the word. Read enough LitRPGs and stats start to lose all meaning.
If you do decide to use stats in a system, you should make it very clear in-story, through action, the impact those stats have. For a stat going up to have meaning, you have to very firmly and intentionally demonstrate time and time again how those stats make what would otherwise be impossible, possible.
You should be very aware of what kind of power scaling you intend for your system as well, and where that power comes from. Is it the skills that propel people into the superhuman, or the stats? How superhuman can people become with the system? Capable of wrestling a mammoth? Capable of punching a planet into oblivion? The larger the scale, the more careful you have to be with big numbers, because eventually every big number looks the same. You might want to be more reserved with increasing numbers, but make each one count more. Or, you could have different 'stages', and when you advance from one to the next, your stat display shrinks down, not actually decreasing the output but simply putting the display into perspective for the stage of power you're on.
I still personally don't think stat numbers is very good, personally, and you shouldn't just replace it with a different-looking but identical in practice system either (like letter grading). It's not the inching forward growth that's satisfying, but the milestones (at least in my opinion, but I don't think it's an uncommon one?), so a system that capitalizes more in milestones will be more satisfying.