I've noticed a couple of things about areas with human enemies:
1) Frequently you clear them out, but then there's a scripted second wave of guys that tend to swarm you.
2) Getting spotted means that they call in a ton of reinforcements.
I'm sure there's some way to get through the Financial District without resorting to gun-fighting, but I couldn't do it even after half a dozen or so attempts.  I can silently clear out all the guys that spawn there initially, but the "second wave" seems to make a beeline for me no matter what I do.
		
		
	 
I noticed something similar and virtually every one of my encounters devolved into a gunfight.
		
 
		
	 
Another thing I've noticed is that the sniper at the end of Summer can see through walls and knows exactly where you are at all times.
		
 
		
	 
This area was literally the last part I cleared earlier this evening.  The first time I played this, I worked my way through every house, which meant dealing with several teams of goons.  On this playthrough, I more or less dashed my way through it and it was a piece of cake.  Granted, it required a couple of tries because I was taking too many sniper rounds to the chest, but I wound up getting through using only a smoke bomb and about half a dozen 9mm rounds.  The key is to just dash past the guys at the end and rush the sniper, triggering the cut-scene.
See 
this for an example of what you're aiming for.  (The guy in the video got lucky in a few spots not to get capped.  The key is his general approach, especially jumping over the firewood at the end and just rushing the sniper).