When are triads used in comparison to open chords?
There is no answer here other than it depends on the player and the context they are playing in. A solo performance by a single guitarist, with or without singing, probably requires something more than triads to carry a song through from start to finish. A second guitar part can focus on triads only, perhaps not playing all the time for the duration of a song but adding flavour and spice at choice moments with short stabbed triads, arpeggiated triads, melodic lines that weave triads and scale notes together etc. There is a good discussion underway on barre chords vs open chords and triads get a lot of attention in there so check it out here: Barre vs Open chords which do you use most often?