OpenCollective is pretty expensive: 10% fees for them plus 2.9% plus 30 cents per transaction for Stripe. (See https://opencollective.com/faq)

Of course, they also provide a lot more services - in a way, they manage bookkeeping etc. for you. So if you want or need those services, they can be very interesting, but if you are just looking for a cheaper alternative to Patreon they are not.

The transparency might be a turn-off for individuals. Some artists on Patreon choose to hide the amount they receive altogether.

You can choose to hide how much you receive.

I couldn't find the source code of Open Collective, at least not from their homepage. Is it like github that says it supports "open source" but the platform itself is not?

There's this: https://github.com/opencollective

Docs here: https://github.com/opencollective/opencollective