Joining a startup that's overwhelmed with demand for its product - particularly if you can see the usefulness of this product yourself - is usually a good move even at 0.1%. Joining a startup that's still iterating on the product (and maybe has a couple customers but just lost a big one and they have to work really hard to sign the next one) may be a bad move even at 10%, because there's a good chance that equity will be worthless.
The article doesn't really mention it, but a great question to ask any potential employer is "What are your biggest problems right now?" Scaling is a great problem to have, because it indicates lots of demand and has solutions that are relatively well-known in the industry. Customer service is a pretty good one - it shows that the company has customers who care enough to want service, and if the CEO is willing to admit this is a problem it'll probably get fixed. Hiring, code quality, internationalization, testing, service outages, brain-dead tech stack, anything that's specific to the problem domain itself - these are also pretty decent problems to have as long as the CEO is attentive to them. Unhappy customers or staff turnover is a caution - you should dig into this further to see why they're leaving, and if it looks like the problem is solvable. Same with financing - many hot companies run low on cash at various points, and you only need to make sure the company isn't going to die, but if the company loses more money than it makes on each transaction that's a huge red flag. The biggest problems (particularly for an engineer) are anything to do with sales, marketing, partnerships, or "growing the business", because the root cause of this is often that customers don't really want what you're making, and nothing short of a major pivot fixes that.
As a side bonus, asking this question is often a big positive signal for the hiring manager, because it shows you're serious about solving problems rather than just collecting a paycheck.
