Matter is not the network layer. You are either talking about Thread (which uses IEEE 802.15.4 like Zigbee) or WiFi. Matter is the application layer protocol.

We have a lot of devices and never have any issues. But yeah, if you put a lot of Tuya Zigbee devices with crappy firmware that flood the network. Similarly if you put a lot of crappy WiFi devices on your network, it's a security nightmare (wise people put them on an isolated VLAN and use WiFi client isolation).