In my opinion, Liara's romance across the series is very poorly written. Her romance across the three games and its DLC are just massive reaffirmation fests. Here is what I mean:-
ME1:- Romance Liara.
ME2 base game:- Reaffirm romantic relationship with Liara. ME2 LoTSB:- Reaffirm romantic relationship.
ME3:- Reaffirm romantic relationship.
The worst thing is that Liara's interactions in ME3 are completely identical whether you romance her or not.
By contrast, Tali and Garrus have romance arcs that are better written across the trilogy. I chose Tali and Garrus here because they are the only 2 non-Human romanceable followers that you as Shepard can have with you throughout the trilogy, like Liara.
ME1:- Tali & Garrus are just your friends and followers.
ME2:- Romance Tali or Garrus.
ME3:- Continue romantic relationship with Tali or Garrus from where you left off. Very little reaffirmation required.
The only thing Liara's romance does better than Tali's romance or Garrus's romance is the sex scenes.
Now if you don't romance either Tali or Liara or Garrus, this is what happens:-
Tali:- Moves on and either ends up with Garrus or is single or is dead.
Garrus:- Moves on and either ends up with Tali or is single or is dead.
Liara:- She stays in love with Shepard in a way that is reminiscent of someone who is clingy and obsessed with someone else.
I would like to be proven wrong on this, based on in-game evidence, not headcannons or fanfiction. Cheers.