I read that UJTs are better for triggering thyristors when compared with R/RC triggering circuits as it has pulse triggering and also has the option to control the characteristics of the pulse delivered.
But
1. Doesn't R and RC triggering circuits also give out pulse waveforms?
2. Plus once the pulse crosses the threshold value of the thyristor, it moves on to the forward conduction mode. So why does the width or magnitude of the pulse matter?

 

If you want a relaxation oscillator to fire the thyristor, then yes Ujt is a good choice for simplicity, if you're looking at getting a pulse from any other source then use a RC filter or resistors.

 

If you want a relaxation oscillator to fire the thyristor, then yes Ujt is a good choice for simplicity, if you're looking at getting a pulse from any other source then use a RC filter or resistors.

You can also use a diac after the RC circuit to generate sharp edged pulses.

 

There is also triac out opto isolators, which can be phase angle controlled.
Max.

 

UJT's are old-school, I have not seen one in decades.