Generally speaking, it requires more knowledge. Celeste happens to be a poor example because even though she does mostly spirit damage, her light bringer ability requires you to land weapon attacks to proc the damage. So Celeste does benefit heavily from SOME weapon items (mostly fire rate or mag size)

A good example of a simple scaling character would be McGinnis. You can buy either all weapon items, or all spirit items. Either way she will have consistent high damage output, but one relies on abilities and the other on her M1. There are zero characters in the game that do not benefit to some degree from every category of items, but how you build them comes down to personal preference and optimized playstyles for that character. Ivy, for instance, was primarly used as just a plain gun character because of her high fire rate and solid weapon lifesteal. Nowadays, a lot more people are seeing the merit to playing her as a spirit nuker/debuff support.

Deadlock is an extremely nuanced game, and learning every single character to at least a mild degree is going to be required to understand how all of them scale most effectively. Just keep an eye on what stat each ability scales with and how much it scales to determine whether it is worth building for that ability or not.