In my experience it depends on the nature of that client. A drawback to having a dominant client is that they have a lot more leverage in roadmap, prioritisation, and product fit. That can seriously impact the internal coherence and maintainability of the product. Too many bits of business logic lodges in weird places because the client didn't want to write custom behaviours, and no time to tidy it up because they're always demanding their next wishlist feature to be deployed yesterday.