Context is everything, IMO.

In engineering work, numbers should always be digits. In prose, numbers should be spelled out.

Breakfast at the Thompson's was a busy affair; 12 eggs and 6 rounds of toast for their 3 sets of boistrous twins.

Compared to

Breakfast at the Thompson's was a busy affair; twelve eggs and six rounds of toast for their three sets of boistrous twins.

To me it's pretty clear which of those reads better and more naturally as prose; digits really 'jump out' on the page, and while that is great for engineering texts, it is incongruent and distracting for prose.

What kills me is when people will mix the two in a single context.

"Between eight and 13 percent"

NO. If you're writing one number in digits, you need to write them all the same way.

Engineer here.

Typically when I type out professional emails or documents that contain numerical values, I write out the number followed by the digits in brackets if it is ten [10] or below for cases of amount, unless I am listing out the counts of items, then I only use digits.

"The updated electrical design will require three [3] new, pad-mount 500kVA transformers to replace the three [3] existing 225kVA transformers,each located on floors four, five, and six."

There are exceptions to every rule. Sometimes it ends up being "between five and 15" which is psychotic.

"One and eight hundred and fifty two thousandths".

Or

"1.852"

You get to decide what's efficient to communicate a specific value based on the criticality of precision and the format of communication.

Like it or not, but peak-compatibility IS peak-efficiency when it comes to language.

Caveat, not including multipliers, like "273 billion"

You mean 273e9?

Any number that I write down is a number. I am not writing novels, the numbers I write down are supposed to be easy you find. You look through the document to find numbers, that is easy to do.

Three and four hundred fifteen quintillion five hundred ninety two quadrillion six hundred fifty three trillion five hundred eighty nine billion seven hundred ninety three million two hundred thirty eight thousand four hundred sixty three sextillionths

Is less than ten

Not just an engineer thing though. Everyone finds it obnoxious.

In nineteen ninety eight The Undertaker threw Mankind off Hell In A Cell, and he plummeted 16 ft through an announcer's table.

00, 01, 10 there i did it

I spell out numbers when I want to emphasize them.

Take George Orwell for example:

"Nineteen Eighty-Four" has a lot more of a punch to it than just "1984."

Next you’re gonna ask me to use actual scientific notation instead of to the most relevant 3 decimal points. I will not use your bullshit centimeters, that’s just 10 mms

For manufacturing I've taken to using spelled out numbers when quantities and names both use numbers. Four 4s rather than 4 4s. Makes it harder for someone to speed through an email and get the completey wrong information.

Zero.
One.

I somehow have "spell out if less than 13" burned in my mind from somewhere in middle school. No idea if it is right, but so far it has worked.

As a mathematician, I refuse to do this.

Nein

Spell out numbers under 10, but not when it's divisible by three or five.