No, because you would need to add structural elements, some way of combining power output, and cooling if you’re aiming for a similar form factor.

And precisely 1/18 the MTBF

In a way, sure. Often when a car engine fails people just replace it. If you were to replace all 18 when the first one failed, the same way you would with a regular car engine, it's not 1/18 the MTBF. And if it was just 1 failing prematurely and you replaced only that one, you also see that same dynamic with car engines.

but it fails gracefully, right? (only one engine fails at a time)

it's like RAID, great until the RAID card itself fails

It's probably most like losing power in a single cylinder; the effects vary from engine to engine.

They tend to seize and when they do and you gang them they'll likely break stuff in the other ones unless you add a freewheel or some other luxury like that.

That's why there's all sorts of widely produced performance vehicles with multiple independent engines!