I'd say aptitude is more about ability to learn rather than anything else. I guess you could call it talent. You could have someone with low aptitude who already knows how to make a bang and someone with high aptitude who has never even heard of Shinsoo; the one with high aptitude would quickly overtake the person with low aptitude even with the initial difference.
The biggest limitation for Shinsoo users early on seems to be the number of bangs they can maintain and control at once. When they had to take the test to evade the Ranker, Quant, he was only allowed to use one bang at a time and this was considered to be a big limitation; one bang means he can only do one thing at a time, reinforce his body or make a projectile, attack or defend, not both at once. You can see the effects of this during the final exam, Bam had issues because he could freeze his opponent but not attack at the same time; he had to release his opponent to attack and they were able to avoid it.
It's weird to think about how weak everyone was at the start, given where the story is currently.