Interesting insight from Aris into the grip of rain tyres and how they're simulated in ACC.
You "feel" more grip because the rain tyres have wider slipangles, everything happens "slower", with "wider" margins and with more time to react.
Monza is a particular track, the slow corners are... quite slow and racing rain tyres have big amounts of grip, even in the rain. So in those slow corners you have lots of grip to deal with.
What catches out always in the rain is the high speed. At high speed, the standing water can cause aquaplaning and this is dangerous. That's why you feel such difficulties at the straights and curva grande. Also consider that Monza has good draining asphalt, this keeps puddles to form even at heavy rain.
ACC does not simulate rain as a simple lower grip surface. ACC actually simulates the rain film over the asphalt. The tyres, depending on their tread, wear, load, temperature and many other factors, can successfully drain the water out and thus giving good grip, or... not. In that situation the grip goes away VERY fast. That's the difficulty of rain driving. It's unpredictable and snappy. If you had just "lower grip" then you would be accustomed and able to form muscle memory... but that's not the case in ACC.
The rain setup is always very safe. To be faster you need to use less TC, but you arrive at a point that it is dangerous or slower... you need to go out of your comfort zone to start catching the AI.
Furthermore, rain in ACC induces rolling resistance and aero resistance. You top speed, especially at a circuit like Monza, in heavy rain conditions, can be 10 to 15 kmh less on par of aero setup. So this means that you arrive at your braking point with much less top speed, thus the not so long braking distance. The best thing a driver has in rain conditions in a GT3 ABS equipped car is... braking.
Finally, I have all the respect for any driver that will give feedback for our sim and Jan Seyffarth is entitled to his unbiased opinion that I listen and see if I can improve, but I also have GT3 drivers that I met and talked with them and with their engineers in almost all the GT3 races like Raffaele Marciello, David Perel, Alex Frassinetti, Romain Monti, and many more that keep driving ACC and telling us that the rain simulation is something never experienced before in a racing simulation. We will try to improve even more.