Scout early game is relatively easy (end-game can be challenging), while Technician end-game is ridiculously strong at the expense of a slightly long build up.
Marine is sort of middle-of-the-road. The Master traits are indeed powerful, but they trade off heavily against the ability to adjust equipment. HP bonuses don't really add up all that much as dLev and difficulty goes up -- t'is better to not get hit than to tank it out. Scouts can use their superior speed boosts to help dodge, while Technicians can build armour that can help deal with this. The Marine is kind of stuck right there -- best way is to try and out-dakka the enemy, but god help you if you get hit. That's why the Marine feels nerfed on N!, everything moves faster and hits more accurately on N! except the Marine.
Don't get me wrong. Marines can hit very hard (see MAD, MAc), but DoomRL has evolved into a game of manoeuvring and position, and being slow can be fatal at higher difficulties. To combat the relative slow movements, I would suggest trading some armour for speed, i.e. Tactical Boots and/or Tactical Armour, but more realistically, A-modded armour/phaseshift. That allows ease of manoeuvring into position, and then one can rely on the heavy hitting Master traits to kill everything within sight before they nick DoomGuy.