It can be, many HCI vendors are sold as appliances which are difficult or impossible to scale unless you purchase more appliances.
Maxta sells a software-only hyper converged solution that allows you to run hyper convergence on your choice of X86 hardware and hypervisor (VMware-centric but supports others as well). With a software-only model you can scale within a node or within the cluster, you are limited only by the capacity of the servers themselves.
Full disclosure I am a Maxta employee.