A few thoughts:
As you say, the thing that's really different about us is exactly that we are focussed on engineers not PMs with our tool. PMs are of course welcome to use it too, but it's a little more technical feeling.
We felt the people building the thing should have the context of usage data and this shouldn't only sit in another team - which was a behavior we saw happening quite frequently when we did user interviews early on.
We started with the fundamentals of product analytics with some features we wanted ourselves but are now focussed on features that are much engineering specific. For example, we are about to release (an optional) "inspect element" for usage data: https://github.com/PostHog/posthog/issues/870, so you can pop that up whilst working on localhost.
We had no idea if that hypothesis was correct - that engineers would care. We did a launch HN to find out and got quite a lot of good feedback and growth.
Re scaling, you are spot on - this is definitely hard! We have users doing 5 million events/day on Heroku's cheapest standard tier dyno. We offer paid support for people who need something higher volume still if it doesn't work well out of the box. We're working on supporting databases other than Postgres as people need them.