I have been doing home automation for almost 30 years. I started with x10, and still have a few x10 devices deployed in my house, so I think I'm in a position to know what I'm talking about here.

There are a few major advantages Zigbee and Zwave have over x10, namely:

• The Zs are fast. X10 takes a quarter to half a second at best to travel through the house and activate the device. If there is noise on your powerline, it takes longer. Sometimes messages are missed. This is old tech, and there isn't a lot of error correction or signal ack.
• Phase Bridge. X10 uses your house power line to send signals. You may or may not know this, but (at least in the US) your power is split into one or more electrical phases. The X10 signals are absolutely terrible at crossing from one phase to the other, and it isn't always obvious when you plug something in which phase that outlet is on. This leads to a lot of troubleshooting. Things like phase bridges exist to solve this, but they aren't terribly reliable.
• Wireless. The Zs don't need to be connected to your powerline to function. There are no powerline x10 devices that run on batteries for obvious reasons. This allows for a lot more versitility.
• No setting house/device codes. If you have ever actually used X10, you know each device has its own house and device code that needs to be selected (usually with a tiny screwdriver while crawling on the floor in the dark, but I digress). This is all handled digitally and is more or less plug and play with zigbee and zwave.
• Encryption. Any idiot with an extension cord and some free time can connect to a power jack on the outside of your home and turn whatever they want on and off with X10. There is no authentication whatsoever. Ironic, considering the company most known for X10 devices sold security equipment.

Now there do exist some wireless battery powered x10 devices (the MS16A motion sensor, for example or the DS10A door sensor). These are actually fairly solid devices, and I still use a good number of them in my home. (I have many DS16As that have been in daily use for more than 20 years)

It isn't all bad, and there is no reason to throw it all out if it works for you. However, if you are buying new gear to automate with, there isn't really a compelling reason to go back to X10. the modern solutions are cheaper, faster, more secure, and easier to work with.

I've never really looked into X10.
A very brief look suggests that the signaling goes over the mains cables? If that's correct, then the big thing for me would be that it separates the communication from mains electricity. Most people don't like messing with 240v, unless they absolutely have to.
Which would mean more needs to be done by an electrician.

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.

@RobotToaster @GreatAlbatross oh man, X10 was such junk. Terrible reliability, expensive, clunky. Always awkward as the units plugged into receptacles with little positioning flexibility, and a restrictive ecosystem with inflexible players tying it up in patents for so long. A couple of hundred bucks I blew there was totally wasted …about 20yrs.