It really depends on how sticky the patrons are, which I would guess is very sticky. A dollar in advertising revenue that Snapchat books isn't as valuable as a dollar that Patreon books for a lot of reasons, but primarily because they have a direct-to-customer relationship and because they generate predictable subscription income.
I would think about the Patreon opportunity as such -
1/ Patreon has built a platform that has proven to be the best way for artisans and creatives to create a direct connection to their fans
2/ Since the inception of the company, they have seen very strong improvements in artist growth, patron growth, and retention in both.
3/ The total addressable market is huge. Every creative is a potential user. Their platform may even be growing the market by allowing people to quit their normal jobs to pursue creative ideas.
4/ They want to raise the money to build new functionality to continue to drive growth by addressing new segments of creatives and improving the product to drive retention.
In summary, I think Patreon has a compelling story for raising money. They need to spend money to grow but not like Snapchat because the consistency of their cash flow is a major reason to continue to invest in retention.