Yes, i agree, ranking subdomain is difficult, but not impossible. Effective link prospecting is required to rank subdomains.
Not at all, I don't see why ranking a subdomain would be harder than a main domain or even a page. What's the reason you think so?
Even though subdomain is considered as another site by SEs, i'd say ranking/link building to subdomain is more or less difficult and i've experimented the same. Let us start from the basics. If someone is doing good/high pr directory submission, the directory rules clearly states that only the root domain is allowed and not subdomains in the URL field. I've seen good/high pr article submission sites disallow subdomain (exception to few). Even when doing blog commenting/guest blogging, many website owners hesitate to allow subdomain URL to be added (exception to few). As i said earlier, even though subdomain is harder to rank than normal rootdomain URL, I wont say it is impossible to rank.
I'm thinking to have sub domain about news to my website can anyone explain why it's difficult if that real ?
You can get subdomains to rank with a bit more work and time, but if you can afford to, use a separate domain name for each of your sites. I use all separate domains, but then I pay for all domains under one hosting plan with Hostgator (I use add-on domains), so you're not paying separate hosting for each domain.