I had SC1 and then SC2 Pro.
If you watch the SC3 video on OverTake and others, they suggest the improvement in FFB quality over SC2 is roughly 10%. At only face value, that might be a fair estimate but, that doesn’t tell the whole story by any means.
Personally speaking, I really like the FFB shaping and Road Texture Boost + the EQ; these have been very effective in adding details in titles that have little to no surface effects in the game FFB. Being able to adjust FFB strength on the fly is really useful, especially when titles don’t offer per-car gain settings.
AMS2: Normally, the Default+ ffb provides plenty of details but, adding a bit of Cornering Effect reminds me of rFactor2 ffb. I also like to add a little Shift Bump effect. Right out of the box, SC3 impressed with finer details that I’d never felt with SC1 or SC2. The tire scrub and brake locking effects immediately stood out in a very good way (high frequency but silky smooth at the same time).
AC Evo: Out of the box, Evo’s ffb can be pretty dull. Since Kunos added the Road Effects slider, it’s better however, the AC titles have always been prone to producing very grainy feeling surface effects. With the FFB EQ in SC Tuner, it’s easy to remove the high frequency details while boosting the softer mid-range ones, or even the suspension effects.
AC Rally: I’m really loving this title but, being able to dial in the road details in the FFB (even on Tarmac) has improved my enjoyment a lot! The game FFB is often cited as a weak point currently but, SC3 has solved that. I also added some Cornering effect which really improves lateral forces on Gravel as well.
BeamNG: Similar experience to AC Rally in terms of adding surface details. Again, it’s easy to shape those details for the firmness or softness desired. This is one of those titles where I like the premise of the game, crash physics, Rally, offroad, etc., but I have not played it much, largely due to the FFB.
I’ve also done testing in AC / ACC / PMR / LMU and others, all with good results. Another effect I sometimes add is the Engine Vibes - usually only felt around idle but, I was surprised how well SC3 deals with very high frequency vibrations (Like F1 frequencies).
The overall Tuner layout and features are very nicely done, imo. The feature switches and additional drop-down tuning menus for filters and parameter settings are great to have. These provided quite a few Easter Egg moments for me. 
Telemetry effects may currently be limited in some titles, so I haven’t yet taken a deep dive into those.