I would say it depends on the opera.  There are such huge numbers of them.
And some are as intellectual as any other classical music in my opinion...
Stravinsky's Berg's.  Do you think that many believe Mozart's operas were
any less intellectual, musically, than his other compositions. Or Bach's
best vocal works, even.  Most opera's are written to show off the voice and
to entertain in a certain way. But I think every bit as much thought went
into them.

Ray Bayles