Saying that TeamCity is their worst product sounds like a back-handed compliment to me. It's certainly not a perfect product, but I've been in multiple jobs where they migrated the CI system (including one that did ~25k builds/day) to TeamCity and it was a significant improvement in scalability and manageability over Jenkins, Travis, and some other Windows-only CI system.
To be fair anything is a significant improvement over manageability of Jenkins.
Oh how I have wasted days configuring build and deployment pipelines in TeamCity. Brings back some arguably bad memories!