I play with Thrustmaster TX and have the same impressions that way to many cars have not the grip they should have, especially with slicks, bad breaks and often oversteer like hell without proper tuning (and some even with). This bad handling out of the box must be frustrating for beginners. So I don’t think, that this game is made for everyone. But the lap times are still quite comparable to real lap times with the same breaking points and gearing and some drivers beat the real cars with racing professionals by far. The game force you to drive proper or crash.
I think, many people have the impression, that the Lotus F1 is to overpowered. It’s so simple to drive and the lap times are fantastic. In this case I can assure you, it is not. This fast lap of Schumacher in 2004 at Nürburgring GP is about 2 seconds faster than the No. 1 in the Forza leaderboard: F1 Michael Schumacher Onboard Pole Position Lap at Europe (Germany - Nürburgring) 2004 [HD] - YouTube
In 2008 they forbid the traction control, so it’s more difficult to drive F1-Cars nowadays, maybe because it was to easy before.
The Ferrari 312T seems fine too, maybe slightly underpowered. Niki Lauda was driving a 6:58 on the Nordschleife in 1975, but the track was 22,8 km at this time, not 20,8 km which was introduced in 1982. But the 2 km more were just two straight lines with a turn, so it’s like a 6:25 on the track today and only 4 people can beat that time in Forza 5. Many lap times are much better in Forza than in the real cars. The real Pagani Zonda made a 6:47 at Nordschleife, in Forza 5 it’s a 6:24. But it’s a very expensive car and Marc Basseng wasn’t pushing it 100% I think.
So maybe the physics in Forza 5 make driving more difficult, but on the other hand it’s more challenging trying to beat lap times of real racing cars and when you beat them without mistakes, it’s satisfaction.