According to whom?

According to Marx, Engels, Lenin and any other respectable communist.

Capitalism is a historical progression rather than something you adopt willy nilly, and it has expanded productive forces significantly allowing us to produce stuff far more efficiently in far higher quality and complexity. With feudalism, it's mode of production was far more individualized, with peasants essentially producing for their and their family's subsistence only, and artisans in guilds would only work in small groups, limiting to what they can produce.

Therefore, this expansion of productive powers in capitalism in theory leads to better life quality, less socially necessary labor time to provide for everyone, less mortality given how we can now produce things like insulin in complex labs, etc.

Keyword is in theory - in practice, everything else in the system goes against that, leads to overproduction and having us proletariat work for much higher hours than is socially necessary, it concentrates wealth to private owners giving them immense political power. That's what communists are trying to do - progress forward so we produce not for profit, but for use based on need which would solve these issues.

Btw, comparison between feudal peasantry and proletariat is flawed - peasants were based in countryside and essentially were the middle class of it, owning a small amount of land that they worked for themselves. Proletariat are urbanized, work in factories they don't own and produce for thousands of people. A more apt comparison in work hours would be proletariat vs guild apprentices - their exploitation and work hours were essentially the same and this system was precursor to capitalist wage labor.

Who has switch over from feudalism been a disaster to?

The unmitigated disaster part existed under feudalism also. Capitalism is slowly turning back into feudalism, which is kinda why it sucks so much now. I hate capitalism, but feudalism was worse.

Fuedalism with a fuckload of democracy might work. But it always turns into a bloodline thing.