What's wrong with awaiting a promise chain?

I’m being told awaiting a promise chain will break things in my code.

Not necessarily, your two code snippets do indeed work the same (as long as someFunction() really returns a promise).

What does it matter which one is used. What dangers does the first snippet introduce that the second one doesn’t?

It's harder to understand and maintain, it's confusing to mix different styles. Confusion leads to bugs.

Consider that you would need to add another promise call at the location of the console.log() call, or even a conditional return from the function. Can you use await in the callback like elsewhere in the function, do you need to return the result from the then callback, is it even possible to return from the outer function? All these questions don't even come up in the first snippet. And while they can be easily answered for your toy example, it might not be as easy in real code with more complex and nested control flow.

So you should prefer the more concise and clean one. Stick to await for consistency, avoid then in async functions¹.

_{1: Of course, there's always an exception to the rule. I would say that it's cleaner to use promise chaining for error handling in some cases where you would use catch or the second then callback.}

Under the hood, async/await is just promises.

That is, when you have some code that looks like:

const result = await myAsyncFunction();   
console.log(result): 

That's exactly the same as writing:

myAsyncFunction().then(data => {
   const result = data; 
   console.log(result); 
}); 

The reason then - that you shouldn't mix async/await and .then chains - is because it's confusing.

It's better to just pick one style, and stick to it.

And while you're picking one - you might as well pick async/await - it's more understandable.