Dogs shouldn't honestly be much of a problem because they always growl before jumping at you. So you'll always know they're nearby and can simply turn and quick shot them without any difficulty. Same with shriekers, you can hear him gurgling and trying to suck up air so you should be able to pop him before he screams at you.
Pretty much every enemy in the game has a visual or audible indicator to let you know they're there so you can deal with them. So if you get surprised by them, it's a good chance it's because you weren't really paying attention or you have your sound off for whatever reason.
And you shouldn't have a combo loss due to lack of enemies on Urban or Mining so long as you're not simply shotgun'ing everything you see. If you space enemies out and melee them all or counter a lot, enemies will constantly keep flowing in and you really shouldn't have to worry about it.
If you're playing duo and constantly running back and forth all over the entire map, then obviously you're stretching enemies out all over the map too and gonna screw up your ability to keep enemies flowing in properly. Stick to an one area, let your partner stick to another, and have at it. If your end dries up, head over to your partner and stick together on opposite sides of that area to filter all enemies to that one area and deal with what comes to you.
The main reason for combo loss is simply bad luck. Such as JUST finishing a melee on an enemy and getting grabbed or hit before you can even move again followed by getting ran over by the napad while being grabbed, or getting knocked to the floor by a stray magnum and then getting hit 5 times in a row from the floor before you can even get up.
Stuff like that is the only real problem in Mercs. It's kinda random and not a whole lot you can do about it sometimes.
I think it's easy to say what it "should" and "shouldn't be"- but in real time- it's is a whole different story- especially when things become very intense towards the end game, U/C in particular. Yes, a dog barks before attacking, but if 5 are all barking, and 2 leaping zombies are coming from different directions- while your partner was just grabbed and you are being fired on from magnum cops and BSAA Agents with magic heat-seeking bullets while avoiding the Napad, that barking cue isn't really very helpful.
"Hey! Porky Pig! I hope you get molested!" PSN: TheWorstType