Simple buck circuit reducing 240V AC to around 5V at 300mA
is there any good reason this circuit wouldn't work and/or be unsafe?

There are a few designs that can do this but I wouldn't recommend a triac type because of peak currents into the 120 mH inductor and the sheer physical size needed for the 120 mH inductor: -

The above regulates to 5 volts DC from an AC voltage range of 85 volts to 265 volts. It can probably be coerced into producing 3.3 volts directly too. The source is here.
Then there is this from Power Integrations: -

Source information here.
These are the preferred methods for non-isolated buck converters and note how much smaller the inductor needs to be in these designs.
$$\color{red}{\boxed{\text{These designs are dangerous in that they don't isolate high voltage mains}}}$$
The preferred method is to use a flyback design not only because the circuit provides inherent isolation properties but, because a step down transformer can deliver much more current to the output for a given (small) transformer size: -

Circuit from here and note the construction size: -

Pretty much all the space above would be taken up by a suitably dimensioned 120 mH in the OP design.
$$\color{blue}{\boxed{\text{There is very little sense in not using an isolating flyback design}}}$$