In an era when cultural institutions are growing and building at an unprecedented rate, arts consultant Adrian Ellis can be counted on for some straight talk. “His sobering message may not be what museums with big dreams want to hear. But Mr. Ellis’s clients say his unvarnished advice is not unwelcome. “He’s a reality check,” said Charles Saumarez Smith, director of the National Gallery in London. “I think it’s healthy for every institution to have a good reality check.”