To truly do it well it's time intensive. That, and unless you do a test with paid search first (or are already getting some traffic with a low ranking) it's hard to tell if certain terms will actually perform well once you get traffic from them. Nothing's more discouraging than spending months to rank for a competitive term and then seeing that the traffic isn't all that good.
the drawbacks compared to what? compared to not optimizing your website for SEO?
if you want your website to get found by searchers, your choices are to a) run a ppc campaign, b) optimize your site for organic rankings, or c) use both ppc and seo to market your site.
i choose option C.
and btw, if you do your keyword optimization correctly and conduct extensive keyword research and constant keyword discovery, you'll never overlook the important keywords. in fact, the opposite it true. by conducting keyword research, you will discover the important keywords.
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