Not saying the level is good, bad, or unrealistic, but:
On my last 2 strat bombing runs, I was pinged by flak. The first mission, it knocked out an engine. The second, it created a fuel leak in my biggest tank, which caused me to shut down 2 engines just to have enough gas to get home. Both missions I was at 26,000 feet over the target.
So, you fly bombing missions enough, there will be some flak hits. Don't really have an opinion on whether or not the flak should be heavier, but it can make a difference occasionally.