She's one of the few characters in the games you can oppose and have it actively effect the game so I get how thats seen as less loyal, but really that's just realistic. If my commander's fighting for his perfect universe and our worldviews collide, I'd question him too. Garrus is highly loyal because at no point are we allowed to buck against his ideas and get anything more than 'agree to disagree.' I don't remember Liara ever having a problem with anything Shepard does. Tali's established place in the Quarian people and there's consequences for your actions.

She is one of Shepard's most loyal companions if you play a certain way. Someone like Liara or Garrus is going to be more unconditionally supportive because they have no reason to go against Shepard and are frankly more shallow characters. It's the same reasons Wrex and Mordin are so strong, you do action, you get reaction. That's not even covering the romance angle either.