I don't know about theory, but the big practical advantage to ZigBee is that it works.
Sorry, that's a shitty thing to say. I'm salty because the only time I tried X10 was 25 years ago, and the experience was less than great. Unreliable switching, spurious commands, slow performance, etc. Sending signals over the power wires sounds great in theory, but in practice there are all sorts of pitfalls, like resistive versus inductive loads, bridging circuits to different legs of two-phase power, or conflicting commands on the wire.
ZigBee has just worked for me, since it avoids all of the potential wiring issues. You just plug a device in, put it in pairing mode, and Home Assistant finds it, interrogates its capabilities, and adds it (by name) with the correct entities. No mucking about with addresses, or adding signal paths to the house wiring. As a mesh network, it's quite robust, since most plugged-in devices act as repeaters.
The downside of ZigBee, of course, is that it may not work well in WiFi-saturated environments, since it uses the same 2.4GHz frequency band.