Has anyone worked with a reliable custom wordpress website design agency? Need recommendations!

Hey everyone, I'm scaling my online service and need a truly custom platform, not just a standard theme setup. I'm looking specifically for a dedicated custom wordpress website design agency. Someone suggested a particular name, but I want to make sure I hire the best fit.

What should I look for in their portfolio to confirm they can handle complex backend functions and custom integrations? If you've had a great (or terrible) experience with a firm that does genuinely custom WordPress development, please share your thoughts and recommendations!