Reviewers with 3 out of 3 failed audits who reviewed a dozen of posts in total are treated the same as reviewers with 3 out of 30 failed audits who reviewed thousands of posts.
No, they're not. Not necessarily...
What happens if I fail?
For most, nothing. You'll be given some guidance on why the post was inappropriate and what you could have done instead, and then allowed to continue reviewing.
Repeated failures, especially in short succession can lead to your review privileges being suspended for a week or longer, depending on the frequency of the failures.
I've been a bit cagey about the exact criteria the system uses to instigate automatic review-bans, but it's a bit more interesting than "fail three audits". Some folks fail dozens of audits without ever seeing an automatic ban, while others get banned relatively quickly. Of course, bans instituted by moderators can be enacted after zero failed audits if they notice you're really just not paying attention...
I'm marking this status-completed, because the system in place is already more nuanced than what you suppose it is, and in some ways more cautious even than the one you're suggesting replace it. That said, there are some additional changes we could make that would make audits less annoying for very active reviewers.
The truth is, not very many people get banned from review, and most who do aren't banned for very long. The message you see upon being banned suggests that perhaps you need a break, and that really is the intent - to give someone who has perhaps been pushing themselves too hard and gotten careless a strong encouragement to step away and do something else for a while.
Indeed, the primary purpose of audits is education not punishment. It's easy to make mistakes, especially after you've been doing a fairly monotonous task for a good while - sometimes, it's helpful to have a sanity check. I fail audits occasionally myself, and always take it as a sign that I've perhaps gotten in too much of a hurry and am in danger of becoming careless; the audit (and countdown timer that goes with it) is a welcome reminder to look up and take a breath before going any further.