I think it may be perceived unrealistic due to (possibly) using time compression which obviously makes them appear much more often. If you went across the Pacific in "real-time" the enemy aircraft encounters would be much further apart so to speak.
For what it's worth;
I draw a 600km radius circle on the map centered on Tokyo and a couple of other possible enemy air "bases". This gives me a rough range limit and tripwire for when I should start being really concerned about aircraft. I cruise on the surface until I reach the edge of these circles. Once inside those ranges I start "watching" for enemy aircraft (i.e. air radar is on and raised to full height).
It's a relatively crude indicator but it allows me to confidently cruise on the surface for at least half of the voyage. It has not failed me yet.
Once I get a contact on air radar, time compression never goes above 32 (otherwise the aircraft is on you before you can do anything). This time compression setting lets the game warn you of a visual siting and enough reaction time to get underwater to safety.
My two cents.
