-> I would like to know why especifically this setup is not reliable, as I've seen some posts about it, cause for me is a great one... but i will appreciate your comments on the problems of it 
It's a good set up, but there are some things to be aware of.As Loyce points out, this isn't necessarily a cold wallet unless you make it so by running it on a permanently airgapped machine. A temporarily disconnected machine is a distant second. Ideally you would run this on a permanently airgapped machine, paired with a watch-only Electrum wallet on your main computer.
Secondly, be aware that Tails has no persistent storage unless you configure it as such. With no persistent storage you are completely reliant on your back up to access your coins. Make at least two, and store them separately.
And lastly, the version of Electrum which is bundled with Tails is not frequently updated. Tails is currently bundled with Electrum 4.0.2. If you want to use the most recent version for any reason, you'll need to download and verify the standalone appimage.