Their pricing page doesn't show anything about usage limits and if it is "per site" or "per domain" or "per project".
Usage limits are pretty soft right now (depends heavily on what you do in your code and how hard you're hammering our servers), but the general intention is to charge you more as you scale up your team, rather than because you got a few thousand users.
So if we build something that becomes wildly successful, we could suddenly be in a world of hurt if our site is deemed too heavy and we're throttled or worse. It would be like Pokemon Go - big explosion, lots of tiny pieces, customers gone.
I urge you to have some kind of professional plan with guaranteed tiers. Otherwise, only naive people or people who really don't care about the future will use it (outside of the prototyping case everyone else is mentioning).
On the other hand, it would be the perfect tool for someone on upwork or the like to do a gig ultra fast, leaving the client to deal with all the future pains. But since most projects end up not huge and successful, that might be just fine!
[heads over now]
My issue is if being used for websites, on average our sites get ~50k visitors a month or even spikes with that amount.
The same goes for complexity of the system, for simple html sites it will not matter much but for very dynamic apps this will require a lot of CPU time.