I haven't heard about any disadvantages of seo. SEO provides good advantages to build your business and online identity for your website. It helps to benefit your business and keeps up your online reputation.
I am an SEO person, and that doesn't mean I can hide the truth about my niche.
To be very frank, SEO is one of the most effective online promotional means. But it is not applicable for all niches (industries/business). At the end of the day promotional strategies should be based on potential Clients, audience and Cost of investment on Marketing.
SEO is not an Universal promotional strategy, and it has its own weakness and strength. There are some niches (industries/businesses) to which SEO will never work.
SEO is somewhat different from many other marketing strategies, in a way that it is subject to rapid changes (both down & ups). Search Engine Optimization, works well for industries who's clients are internet savvy or at least use internet more often.
Advantages of SEO:
Highly Affordable
Highly cost effective
Highly targeted
Brings added Trust (from SEs)
Increases Reputation
And more
Disadvantages of SEO:
Takes Time
Subject to SE updates (Google updates)
Subject to Competition
Should be Managed
On the whole, if your can wait for some time, SEO is a perfect strategy, but if you are looking to sell Tickets for World Cup or Premier league the PPC would be far better option as SEO takes time. However SEO is highly cost effective and lives longer than PPC, where you have to pay for every single click (visitor).
Advantage:-
SEO Gives Business, generate traffic, Conversion, SEO through improved your business growth.
Disadvantage:-
Risk penalized, update will come, SEO no full control, SEO take more time for ROI, no guarantee results.
Advantages are that it helps a site to get to the top of the search engines for its targeted keywords & to increase traffic to the site. The disadvantages are it is a long term process and it really take lot of patience whoever is doing seo for the site.