24V AC to me seems like a strange choice when using DC-DC power supplies allows for using any external adapter, including USB PD, without the need for a loose transformer.
Also, when a plunger gets stuck, the coil continues to draw a lot of current, and could burn up. This is a failure mode of solenoid valves in industrial processes. You can get DC solenoid valves, but the coils are bigger and they're more expensive. It means you shouldn't run an AC solenoid on DC unless you're really thinking hard about what you're doing.
And speaking of failure modes, another amusing feature of solenoid valve systems is the sheer number of ways that they can fail. One thing that's always counterintuitive to beginners is that a valve or its control circuit (or firmware) can get stuck in the "on" position. This is true of solid state and mechanical relays. A well engineered system plans for this possibility. "Stuck" could mean releasing a lot of energy (e.g., if controlling a heater or motor), or flooding your garden, or your house.
Also, AC is easier to switch with physical switches, and there's a ton of stuff still using physical relays at low voltage. It would be nice to go DC but there seems to be inertia.
The exception is 'dumb' solenoids, valves, pumps etc, which are actually powered by the 24Vac.
We also rate AC voltage by RMS, which (for resistive loads) is the voltage that would transfer the same amount of power as the DC equivalent.
But really the peak voltage is higher, so we need parts with a higher rating despite the fact we are not using that peak the whole time.
And then there's issues with power factor. Modern gadgets don't draw power in the nice sine wave pattern without extra tech to make that happen. The extra tech is common now, but isn't needed with DC. Without PFC, you get more line resistance losses than you want.
But now we have solid state switching, and it's cheaper to convert DC with a SMPS than a bulky transformer.
We also don't use long series circuits commonly in anything I know about, except for Christmas lights, which are mostly LED now and work fine as they are(As long as nobody forgets a smoothing capacitor to get rid of that horrid flicker the cheap ones have).
Series circuits have the same current everywhere, but modern stuff is constantly changing the current it draws to save power. Which would be an issue if one device wants to be in sleep mode but the other wants to be full power.
I suppose we could do series circuits with constant current power supplies, so the supply pushes a constant 1 amp, and the devices alter their voltage drop to draw more or less current, rather than altering their current draw.
AC seems to be more suited for higher voltages(Where arcing is a problem) and analog stuff(if you don't want to use SMPSes and you have resistive loads to drive).
AC also has WORSE voltage drop than DC because it exhibits something called the skin effect that causes the middle of your cable to not be used as much to carry current.
Tesla triumphed over Edison because we didn’t have the technology to step between voltages back then. DC transformers are still incredibly expensive. AC transformers are coils of wires.