This coin is good investment, but using TOR for everything, can someone explain that benefits ?
There're lots of benefits. This short DeepOnion introduction video explains only a small part of what DeepOnion is offering:
https://www.youtube.com/watch?v=KDtl-1SVtpY100% guaranteed privacy and anonymity, untraceable payments, multiple layers of security and so much more.
Quite some time ago I read about some backdoors in TOR. Could that ever become an issue or is that problem already off the table?
I am no expert but from my understanding even without Tor, DeepSend and DeepProtocol will still be 100% anonymous. Tor just anonymises your wallet connection while the other stuff anonymises sending and receiving separately. If you are just sending normally and there is a Tor back door then potentially you won't be anon, that's a big if though
there are different layers of anonymity
Tor provides network level anonymity, physical location of nodes protected.
DeepSend is blockchain level anonymity. Blockchain records hided by mixing them
DeepProtocol (Stealth addresses) protects only receivers ONION wallet address so you don't have to give real wallet address someone you don't trust)