Ask HN: To maximize SEO, should I merge my personal projects on same domain?

In this week's post on instapaint, he says and shows that using custom projects on his main domain helps his company's main home page to get more views. He says it makes his website rank better. I understand this might not be the case but it made me rethink how I display my stuff online.

Let's say I am running 10 quality websites on different custom domains. Some get 100K uniques per month, others only 100. Money is not a problem.

I personally think using a custom domain for each is more attractive for the user. This might not be the case. I'd like to know what are your thoughts about this.

1) Considering only SEO, should I merge all projects on the same domain? example.com/"project_name"

2) Is domain authority more important than not having a related domain name?

3) Could a "bad SEO" site bring down other sites on the same domain?

4) What would you do? Any other suggestions?

Thank you for your help.