QA/QC is very expensive, especially for low volume. You send something out to a customer, they claim its broken, you ship out a replacement at your expense, maybe the customer eventually sends back the broken one, then it sits on the shelf for 2 years because you don't have the manpower to do a root cause investigation. Now you've sold 300 units and have replaced 50 of these and you have no idea why they're broken, what went wrong, and have to a a full validation of each one from scratch. Most of them seem to work fine in your test lab (maybe it was the customer's fault? But maybe your design is incompatible with their production environment?) and the rest all have unique problems which each take 1-5 full days of investigation to nail down a root cause.

Maybe 2-3 share a single root cause, and it's not clear how you'd prevent this from happening in the future.

If you're shipping large volumes, this can add 10% to the MSRP, but if you're shipping low volumes, $50-100/hr of weeklong diagnosis and investigation can easily add 50-300% to the price of each unit.

And that's if everything else is going perfectly. Which means you lucked out and found an amazing Chinese contract manufacturer who works with you hand-in-hand to fix any design bugs and manufacturing issues, and ensure parts availability.

The math is this: BOM + cost to acquire a customer + cost to sell the unit < sale price

Otherwise you don't have a business. Please don't ask me how I know this.

10x is a rule of thumb, YMMV. You just need to make the above inequality evaluate to true. You can (maybe) raise money to deal with the various non-recurring expenses. Oh and you should pay your employees too.

NRE into the production line, especially hard tooling for plastic is huge. Plus all those things you mentioned, especially inventory risk are often underestimated.

Don't forget that any manufacturing run itself is a huge risk. Tiny changes that need to be made to address anything from RF performance to fit and finish can invalidate a run and just burn up hundreds of thousands to millions of dollars (depending on scale)

Supply chain into consumers is brutal: retail outlets mark up products by 100% (ie you sell them something for $100, they sell it for $200). If you use a distributor, that's another 100% markup. 10x BoM on consumer price means that some of your biggest sales channels to consumers (BestBuy, Walmart, Target) are paying you 25-33% of what they get from customers. To get your own markup, you need 8x BoM.

The probability and cost of failure? And/or alternative revenue streams with lower volatility available to people with the skill to pull it off.