If using FaaS can be a quick bandaid solution to get you up and running without worrying about having to design your own equivalent of the fail whale, then a certain degree of lock-in might be worth paying.
FAAS (Or, MSA) adds a ton of complexity right off the bat. Why take on that risk when you have little money and less time?
There is a reason Amazon gives away a lot of free credits to startups, and it's the same reason drug dealers give away free samples. If the economics are right, and growth is strong then you might be okay - but you also don't want to find yourself with a $100k/month AWS bill in a startup with only $2m of annual revenue.
(P.S Greetings from the other side of the Spree. I love La Lucha on your street!)