Technical requirements for a SaaS app have a tendency to become something like a Jenga tower over time. You'll be able to sail through launch and initial customers but then slam hard into difficult architectural challenges as you suddenly onboard larger customers or unique use cases.

For SQLite my guess would be areas of high concurrent write throughput - like a seasonal holiday/rush, a viral influx of users, or the onboarding of a large client.

Its not that SQLite can't handle these situations with careful architectural decisions. Its that out-of-the-box solutions, like the kind people depend on to solve business-issues in short time frames, won't support it as readily as more mainstream options.