I have yet to ride a Vekoma SLC because I have not yet visited any parks that have one. From everything I've heard and read, SLCs are notorious for being very rough rides, and I'd like to know exactly why that is. With the exception of the inline twists, the transitions don't really look bad. It could also be that the wheel assembly might not have a good grip of the track. I guess it could also be that some enthusiasts are oversensitive to roughness or that they let just a few jerky moments declare a coaster as a torture device. However, those are just guesses and someone that has ridden one will be able to answer this question more accurately.
As someone who has ridden the classic SLC - Mind eraser SFNE, the ride is indeed very painful, your head is literally lodged in heavy padding from the restraints and its extremely jerky, you tend to get a very bad headache from hitting your head against the 'padding'.
From what I've heard, it varies between slc's. Some give good rides, and some (I've heard the one at Heide Park is regarded as one of the worst) give really bad rides.
I've ridden Infusion @ BPB (originally Traumatizer at Southport - built in 1999) and Kumali @ Flamingoland (2006). So one standard layout and one shenlin layout.
Infusion had a few rough spots - especially the rollovers but as with Mrcrolly it was caked in padding. Kumali was pretty smooth though - no complaints from that ride!
I've ridden Blue Tornado in 2013. The biggest problem is the restraint. You hit your head all the time. And if you try to lean back to prevent your head from hitting the restraint, you end with a headache from the vibrating seat.