SD Express is fundamentally PCIe + NVMe in a different package, so the technology is prepared to hit incredible speeds when the fastest flash gets physically small and efficient enough to fit.
But I assume there is controller, power and heat limitations. You can only fit so many NAND package within an SD Card which limits its speed. Then higher speed SSD also has power / heat issues. And with that limitation I am thinking of reliability compared to M2 SSD.