What makes this perfect ensemble possible is years of rehearsal (preceded by even more years of the individuals achieving technical mastery of their instruments). String instruments have an advantage because a normal bow attack is not nearly as sharp an attack as a note on guitar, for example. Here is the LA Guitar Quartet, a highly-accomplished ensemble, with a Rossini overture:
You can probably hear the difference. The guitar quartet cannot possibly sound as unified in ensemble as the string quartet because of the nature of the instruments. A guitar has a very sharp attack and the sound begins to die away immediately so your attention is always drawn to that attack. Getting four instruments with such an extremely sharp attack to sound absolutely together is so difficult that what we usually get, as in the clip above, is merely a pretty good approximation. The string quartet has the advantage of the softer attack of the bow. If they were restricted to playing pizzicato, I think we would hear a few more ragged entries! How about another ensemble? Here is the Debussy Sonata for Flute, Viola and Harp:
Here the ensemble is even easier because we have three different kinds of attack. The harp is quite sharp, like the guitar, but the viola has a softer attack and the flute is in between. Because the beginning of each note is so different on the three instruments, we don't even notice if they are perfectly together or not. Listen to the first phrase the flute and harp have together. Are they really together, or just sort of? Does it matter? But put two harps together and you start hearing that crunchy sort of sound indicating that the attacks aren't quite together. Here is a Strauss waltz with two harps:
Occasionally the problems can be overcome with adroit compositional choices and some really superb musicianship. Jakob Lindberg and Paul O'Dette demonstrate this with Renaissance lute duets. Lute duets were quite popular in Elizabethan England and composers wrote textures where the instruments answered one another. Instead of having to do the same thing at the same time, where any lack of coordination would be immediately evident, the instruments tend to echo one another, which actually adds to the musical charm. Plus, these two lutenists are outstanding ensemble musicians. Have a listen:
I have done quite a lot of ensemble playing with flute and voice and in both cases, after spending enough time in rehearsal, I no longer have to watch the other performer very closely. Why? I can hear when they are about to being. I can hear them taking a breath prior to launching into the phrase and that's all I need. It is actually more reliable than watching them. Here is an excellent example. The Entr'Acte by Ibert begins with both instruments together and you can hear when the flute is going to begin:
Every ensemble has its own quirks, strengths and weaknesses. When I played in a rock band very early in my career we first had a drummer with the unfortunate tendency to slow down in the chorus. Alas, the drummer we replaced him with, while having a much steadier beat, had a problem with the stand for his snare drum that involved him having to stop drumming completely from time to time, to repair it! But I would still prefer a real drummer to a drum machine which never wavers, never makes a mistake, but also can never have any musical expression... Now here is some excellent ensemble in a rather different genre:
Now that's cookin!