I used TFA Smooth a lot when I started mixing, based on various recommendations. Not recently though, and I don't recall ever trying the same recipe with and without Smooth to see what the difference was. I still have a bottle of it. From what I've read it seems to be used for the same purpose as Marshmallow, which I've been using a bit recently. Are there differences between the two?
Yes. The big difference is TFA Smooth has Triacetin in it. Over time Triacetin will cloud and crack plastic tanks. Same thing with FA MTS Wizard, which does the very same thing as Smooth.
Either one of those (besides reducing harshness) can also help to blend (versus layer) flavors in a mix. Nearly any sweetener, when used imperceptibility, can help to do the same (blend).
My preference is to use a cream, a vanilla, a custard or a marshmallow (many times a combination of) to take the edge off a harsh mix. Those will also thicken, and enhance, nearly any mix.
Some fruits are harsh. Using another, complimentary, non harsh fruit flavor will help to minimize harshness.
Using a higher VG ratio, and/or lower nic mg, will also reduce harshness.
Some confuse harshness with throat hit. Some like a strong TH while others don't.
Wayne at DIY or Die has an excellent webpage, and video, on the topic.
Why is My Ejuice Harsh? (Updated) - DIY OR DIE Vaping
@Zazie can you please add this to the Tips section. Thank you muchly!