People aren't saying it's that hard. Any car is easy on your own. The Sauber isn't especially difficult in it's own right.
The trouble is that in a race you try and keep up. You over drive occasionally. You try to pass people when the pass isn't REALLY on (e.g. T1 Syndrome). You expect people to give room and drive cleanly. The Sauber is just too quick for the majority of people to do that with.
I know I'm not the best at LFS, but I can hold my own in most car/track combinations. And I've raced the Sauber against people whom I rate as strong or stronger than me, and the races were awful. The slightest misjudgement, the slightest bit of lag, and there is a race ending crash. Over time people learnt to calm down. The follow until a pass was well and truly on, and to back out of ones that weren't. But the majority of people don't do that.
In real F1 you have the best drivers in the world, racing the best cars, with real life risks. Thus the racing is generally devoid of lots of passing, but also devoid of accidents and setup exploits, or a range of driver abilities. All (bar Sato) don't tend to do really stupid things in T1. All get within 0.5secs of their cars potential. LFS drivers can't do that, and thus the racing isn't satisfying.
The only type of Sauber racing that would be any good is a top-notch league with GP length races (~60/70 laps), where points count, results count, tyre wear and management count, strategy counts etc. It's just not pick up and play friendly like even the FO8 is (which still needs longer races imo to unleash it's potential.
The FOX, the UF1, the FXO, the MRT etc all work as pick-up race cars, but not the BF1.
As LFS progresses and everything from setup options to tyre heating to brake temps to strategies etc then the BF1 will come into it's element, but right now for what 'the masses' want it's just too much.
My first ever lap in the Sauber was a 58 second lap of Blackwood with Race_S (okay, the second lap, as the first was my outlap). It's not hard to do, but that in no way qualifies me as able to race it properly. Only when you can do sub 55 second laps for 50 laps without any mistakes, whilst following another car, and putting in faster laps only when it counts will you be good enough in my book for that car.