Liara can be your most powerful team mate if you are a biotic because of her extremely fast cool downs on both Singularity and Warp. Liara does not work as well for an engineer because of the more limited combo possibilities. With a Sentinel or Adept you can Singularity>Throw almost as fast as you can identify targets (assuming they are unprotected). With a Vanguard, the combo would be Singularity>Charge. For an Engineer (and Infiltrator) you would mostly be using Liara's Singularity for crowd control. While her Warp can detonate fire/tech combos, its cool down is at best 5.42 seconds assuming you spec her for maximum cool down in both her passives and Warp, and put her in recharge armor. By way of contrast, her Singularity can be can achieve a cool down of less than 3 seconds, though I prefer evolution 6A rather than 6B in her passive, which gives a cool down of 3.1 seconds.
You are of course correct, though personally, I find paring of adept Shep and Liara almost a waste. Or rather... I don't know, too much biotic supremacy
Never got that feeling with vanguard at least. Yes, Liara and adept synergize almost perfectly, but when I was playing as adept, I found myself taking Liara along on the missions much less than with other classes, since I could do most of her work myself.
I'll admit I'm biased, since I love engineer the most of all classes and despite what you said, I still think Liara and engineer have a good synergy. Liara takes care of battlefield control and Shep takes care of explosions and hacking. Then again, engineer is my canon Shep and I just love the idea of a tech/biotic duo, who can rip everything before them to shreds. And Liara having Warp and being able to detonate fire and ice explosions and tech bursts is quite enough for me.
So yeah, I agree with you and everything you say, but still stubbornly insist that engieShep and Liara make a great duo 
Liara's draw back is her very low health. To compensate for that you need to make sure you are ordering Liara into cover somewhere behind you while Shepard takes point. Do not let her run off and do her own thing. Her AI has too much Krogan in it for her own good. As a Sentinel you can increase Liara's health and shields by 30% by taking the 5A evolution in Fitness.
Yeah, she really starts showing that krogan side in the third game, that much is sure
Sentinel is the only class I haven't played yet, so I'm interested: would that ability to buff the allies combine with the one of Vega? I believe his evolution gives the squadmates 20% more health and shields, but it never shows in their character screen, only on James's. Together with sentinel, we're talking about 50% increase and that's quite massive.
And speaking about this... how much more powerful would Liara be if she had Throw? Say, instead of Warp Ammo? Don't get me wrong, WA is great power, but Liara's weapon damage is almost negligible, all things considered. I don't think WA helps her that much in that aspect. But if she had Throw... she'd have a detonator for all of her powers and one with almost no cooldown worth mentioning. In fact, I'm pretty sure developers didn't give it to her just so she wouldn't be too powerful. There'd pretty much be no reason for any other squadmate to be there 
Any thoughts on this?
And after all of that, have some pictures of Liara. Some awesome looking Liara first:

In MShep flavor:

And FemShep flavor as well 

All of them by http://the-joeblack.deviantart.com/