So I tested the full extents inputs the way you described while sitting on the ground with flat pitch.
I believe that I am seeing the same response that you are.
If I push the cyclic full forward, it rolls to the right.
If I push the cyclic full right, it tilts backwards and twists to the right.
If I push the cyclic full back, it tilts straight back.
If I push the cyclic full left, it rolls slightly to the left.
I don’t know if this is how a real helicopter would respond, since I have never made a full travel input while the blades are spinning. I’ve only ever made full inputs during the before engine start checks to make sure that the controls are free & correct and not binding.
In a real helicopter, with flat pitch (collective full down) the body of the helicopter doesn’t tilt or roll with cyclic inputs the way that it does in the sim. Only the disk moves…but again, you would not make a full travel input like that while the blades are spinning because you could damage the main rotor bearings. So I’ve never done it, so I can’t confirm what would happen, nor do I ever plan to. When you pull up on the collective, the blades cone and aren’t traveling in the same plane as they were in flat pitch anyway. This is by design. It’s quite possible that the rotor could contact the tail boom with a full cyclic input at flat pitch.
Anyway, I continued to test, and the helicopter responded the way that I expect. I recorded this last test and you can see my control movements in the input viewer. I believe that you are expecting the helicopter to respond the way that an arcade flight model would, and that’s just not how real helicopters respond to control inputs. I was able to throw the S300 around in a way that I would in real life, and it handled as it should.
The S300 from Nemeth is, in my opinion, one of the more realistic helicopter flight models in MSFS. It’s probably getting bad reviews, or a low star count because most people don’t know how real helicopters behave. Helicopters are challenging to fly. Only about 5% of pilots IRL are helicopter pilots. It’s not for everyone.
I didn’t test with the assists on. I’ll try that next…
Edit: I tested with the assists on, and it still handled how I expect, but it felt numb and sluggish. Still more lively than other helicopters in the sim though. You can’t just pull up the collective and sit in a stable hover without cyclic inputs like you can with other helicopters. Anyway, I can’t find any issues that would deem it unflyable.