If SEO is done properly using white hat tactics then yes, it is very safe. Problems can come into play when black hat tactics are used. Black hat techniques are tactics to rank that go against Google policies. And while they may work for a while, at some point a core update can at the best seriously lower the search rank, and at the worst give the site a penalty which will remove them from the search engine results. White hat tactics are ones that do not break any Google policies. They take more time, but the search rank growth you get is sustainable.