Linear supplies can be adjusted with external controls
That's possible with both switch mode and linear power supplies.
But the real reason linear PSU are used less is because they are inefficient, and thus, waste power in many ways. This adds up, and there is a worldwide movement to reduce the quiescent drain of consumer PSUs. SMPS are noisy but efficient, and many devices are designed to suppress or can live with the noise. Linear supply losses occur from the small quiescent current leaked by the regulator, heating from various sense resistors and eddy currents in transformers etc., so even when not used, linear supplies draw a higher quiescent current when unused, but powered ON, so current flows in the primary coils.
Mains powered SMPSes leak more current from the mains side to the DC side or earth than linear supplies because of the Y capacitors.
Switched mode lab quality power supplies use a linear regulator after the SMPS to clean up the ripple.
Correct me if I am wrong here...
Based on the Wikipedia article, if you have a 240V source attached to a 1/20 transformer, the LV side of the transformer is actually safe to touch since the current would be about 6-12mA which is enough to feel, but not enough to kill. So, what you really have to worry about is just the live mains wires and the possibility of shorting them to something you might touch, but anything after the voltage has been stepped down is actually safe?
I doubt you'll get as much as 6 to 12mA through you from a 12V transformer, you'll probably get less than 1mA which you won't feel. The article gives the internal resistance of the human body, once the impedance of dry skin is added into the equation the resistance can be over 1M.
There's no definite safe voltage. It's current which kills not voltage but due to Ohm's law current can't flow without voltage. The lethal current will depend on its path through the body, the age, gender, size and health of the person and whether it's AC or DC. Mains frequency AC is more dangerous than DC but the risk of shock diminishes as the frequency increases as the nerves don't respond to higher frequencies so well. Above around 20kHz the shock risk diminishes altogether but high currents can still cause death and severe injuries through deep burns caused by heating (Ohm's law P = I
2R). The lethal voltage is also determined on the resistance of the human body and skin which dependant on the thickness of the skin and the moisture.
Standards vary from jurisdiction to jurisdiction. Here in the UK, appliances run from an isolated supply (isolating transformer, battery, or SMPS with the secondary not connected to the mains or earth) with a voltage under 120VDC or 50VAC don't need to be earthed, this is known as class 3 protection by SELV (Separated (i.e. not directly connected to earth or the mains), Extra, Low, Voltage). In dry conditions voltages below 60VDC or 25VAC don't need to be insulated from the user. In other conditions (i.e. outdoors) the voltages are halved to 30VDC and 12VAC. As you can see, the safe AC voltages are lower because mains frequency AC presents a greater shock hazard than steady DC.
Another thing which protects you from shock is the fact that the secondary side of the transformer is not connected to earth so you need to touch both wires in order for current to flow through your body. It wouldn't matter if the secondary voltage of the transformer was 230V (i.e. an isolation transformer) you still wouldn't receive a shock unless you touch both wires, assuming of course the insulation between the primary and secondary is good enough (it will be) and one of the wires hasn't become connected to earth (which won't happen if you build it in a plastic enclosure and don't have an earth wire).