Zigbee2MQTT does not scale better or worse than ZHA when using a single Zigbee Coordinator. Scaling on a single Zigbee Coordinator is all up to the specific Zigbee Coordinator adapter and firmware used as well as the Zigbee Router devices you add and their placement in your home. As mentioned, Zigbee2MQTT and ZHA has normally no control of your Zigbee network mesh as that is all done onboard the Zigbee Coordinator and handled by its firmware, so scaling on a single Zigbee Coordinator generally has nothing to do with the application level of your Zigbee Gateway host software (unless you enable ”source routing” which is normally not done on modern Zigbee Coordinator adapters with newer firmware).
Regardless of what solution you decice to do with, read and try to follow all the tips here → Zigbee networks: how to guide for avoiding interference and optimize for getting better range + coverage