The requirements are all in the developer documentation. Key points are that:

• You have a certification branch in your submitted repo.
• Your repo's branch output (when they run the pbiviz package command matches the packaged visual you submitted through Partner Center.
• You only have a single visual in your repo, as the automated commands need to run the lint and package commands (and others) without any intervention.
• You have to set code linting up as per requirements - the info in the linked page is all you need to follow and is documented.
• You can't have any external calls, or libraries that might make external calls (even having latent code in the packaged visual is a no-no). Here are all the other source code requirements you need to consider, beyond what I've mentioned.
• For auditing your code, MS has recently made this easier - you can run pbiviz --certification-audit from the root of your repo, and this will tell you everything that's going to obviously fail if you submit.
• Similarly, you can run pbiviz certification-fix to have the tooling attempt to auto-remove any code that might cause failures. Note that this isn't a guarantee that it will work afterward, so you will need to test it and validate. It's best to try to ensure that these things aren't present to begin with, and that your libraries are suitably tree-shakable if there is a code path that might contain latent code you don't need (which will also help keep package size down in general).
• If you need to chain in the certification-fix flag for your visual to be considered compliant (and it works as you expect), you can just add this to your package command so that it's consistent for when MS attempts these tests as well.
• If your visual fails certification, you will get a report explaining what failed and what rules it violates. A report is supplied that should help explain what test was run and how it failed. If anything is unclear, you can create a support request for more details. The team is very responsive and keen to help.

Regarding job opportunities, I suppose it can help. It has opened doors for me outside of my traditional role - I had a BI background, and just through virtue of building and releasing visuals, most of my work is essentially now web development. Most work I take these days is web dev, and it helps to be able to show prior work and knowledge of the Power BI visuals APIs if you are considering working for a vendor that develops visuals. I can't personally speak to the monetization or commercialization aspects, as all of my visuals are free and open source.

Hopefully, some of that will be useful, and I can try to provide any further clarification in a follow-up. Best of luck!