The danger is loosing the natural horizon.
Quite likely on a hazy summer day over the sea. An inexperienced pilot will end up in a spiral dive if he is not current on instruments. The airplane has a natural tendency to end up in a spiral dive if left alone.
Once a spiral dive is allowed to develop beyond 45° bank, the poor pilot will see the rate of descent and try to stop the descent by pulling back on the stick. This will make the spiral dive even worse, etc etc.
There was an accident near Ostend where a Nord 1100? (Me 208) was flown into the sea with sightseeing passengers on a hazy summer day. There was also an Air India 747 lost in a spiral dive accident.
Dirk