Obama did not warn Trump against hiring Flynn.  He suggested or recommended Trump not do so because there were more qualified candidates for the crucial White House post.  If Obama had more information than that about Flynn, he should have made it known to Trump.  Yates, on the other hand warned the White House about Flynn six days into Trump’s presidency because she had specific information that could result in a possible danger, problem, or other unpleasant situation.  Armed with the information provided by Yates, the White House responded by further investigating the matter, found that Flynn had lied or mislead the VP about his conversation with the Russian Ambassador and for that he was shown the door.   Based on the facts and circumstances reported in the media, the Trump administration acted appropriately in this case.