The "attenuator" in the Katana and Catalyst are most likely not much more than just a global master volume control for conveniently setting them to a range of volume that is most suitable for the situation. There is absolutely no need for this in a Fractal based rig because you can just control the overall volume with the output volume knobs on the Fractal.
Cab sims will never sound the same as a guitar cab in a room. They are a representation of what a mic on a cab would hear. You can mix cab sims together to tailor this sound and adjust them in various ways. This allows you to pair the perfect cabinet simulation with each amp model. Otherwise you will notice that different amp models don't sound quite as distinctly different. This is a phenomenon you can hear with real amps and cabs too. It takes time to get used to and learn how to work with cab sims.
In that sense your Katanas and Catalysts are more straightforward as they have just that one guitar speaker, which is often a jack of all trades, master of none to make it work decently enough for the different amp models in the unit. It's a compromise. The Line6 Powercab attempts to be more like changing the speaker in a 1x12 or 2x12 to different models. I haven't tried it so I can't say how well it works but experiences are very mixed. Some love it, some don't, some just use it as a fullrange speaker because the speaker modeling does not work well for their preferences.
The benefits of studio monitors in a home situation are that they are relatively compact, don't need to be played very loud to sound great and you get full stereo sound. The stereo makes a big difference for many effects. Whether you put the monitors on a desk or not is up to you. That's just the convenient place for most people as they often double as speakers for e.g mixing your recorded tracks or just watching YouTube videos.
You will see people unhappy with pretty much any guitar product ever made, even if it's just a one knob pedal. In many cases it's user error and more about what else you plug into it. Just like you would not plug a world class tube amp into the cheapest, shittiest guitar cab you can find, you don't plug a Fractal into the shittiest fullrange system you can buy. I have no love for those cheap Headrush units for example.
That's why earlier in this thread I recommended that you buy a FM3 + good studio monitors over spending the same money on an Axe-Fx 3 alone. While the Axe-Fx 3 is more capable than the FM3, it's all moot if all that great tone is mangled by a bad output system.