Well I have a few complaints regarding at least some new gear. That is some weapons and weapon mods added to the game have outright made some weapons vastly superior to that of others and really not worth getting.
For example, the Car-4 was always the sort of overall good weapon, but with new mods added it can outclass almost anything except perhaps in terms of power. With the right mods you can get a weapon the can have a high stability with high accuracy and decent power. I mean even with the recently added mods I for other guns don't see people using things like the UAR,the Gecko, the JP36 ,the AK5, Eagle heavy rifle, Commando 553, etc. Hell the one gun I want to love, the IZHMA/Saiga shotgun is vastly underwhelming and really only unique feature is that fact that it has a magazine, but with sub-par power coupled with a dismal accuracy and stability and no real mods to make it feel better just leaves a bad taste in my mouth (and where the hell is my damn drum magazine attachment for it? I dont want to spend perk point on it god dammit). This isn't even covering all the secondary weapons in game that I think makes other secondary useless.
Apologies that I got off topic, and don't take this as me hating on the game. I freaking love payday 2 and have 670 hours in it and will continue playing it further, it's just a game that comes so close to that dream game I want but does things that irritate me because of some design choices.
Anyway again to answer "Give me one legitimate way in which any of the new content, paid or free, has negatively impacted anything that was there on Day 1." Well with recently added guns/mods (most of which are behind a paid DLC) make other mods and guns worthless and there is no sense of balance between them, just some thoughts that went like "hey we made a new gun for the game, let's just slap on some good stats and put it in" without thinking how that might impact other guns.