Is a composition good when is played what is sung?

In fast and rhythmic passages yes, from time to time the orchestra and soloist(s) can play/sing the same melody together.

But for slow and medium-speed solo parts where opera singer's interpretation is important, it's better to keep accompaniment in the orchestra.

Attention: I just wrote it here to give a basic idea. What I write is not a set of general and accepted rules, I know that there are works to which the opposite is true. But it takes more skill and experience to design, compose, arrange and perform them.

Surely both approaches can be used. If an instrument plays the same line as the singer, then providing the instrument is quiet, the singer might be able to get the pitch more accurately. On the other hand good singers might not need that level of support, and having different lines for instruments would not over power the singers. Keep looking at works by other composers for examples.

Another possibility is to share the vocal line between several instruments, so as to provide variety. There must be several different ways to do this. Some composers might have an instrument play the vocal line before the singer enters, so that he or she can get the pitch etc. Many composers wrote for specific singers - there must be several different ways of managing this.

Writing complex parts for singers to sing over could be very taxing in long works - think Wagner. Some singers might like the challenge. Other composers might write much less strenuously demanding music.

I think it just depends on the sound you are after. Experienced singers don't need the support of some other instrument on their part. In fact, in more modern works, it is the job of the conductor to follow the soloist. That can become harder if there are other instruments to try to coordinate.
All of which is hard to write in notation software. You can't get a good feel for how things sound.