The older type of nylon bristles are a bit rough. Newer synths have options how soft you want them to be. Some feel like lathering up with a kitten on your face, if you can imagine that.
I'm not sure of the generation and the corresponding numbers, but for the ones I have tried I presume it's along the lines of
Older - Omega S brush fibres - tips aren't the softest and the fibres are springy, difficult to splay
Newer - Plissoft/Tuxedo etc - much softer, splays well
Newest - 'Next Gen' Knots eg Muhle etc - just as soft but with better performance than the previous generation
I have a few Yaqi brushes in the Plissoft/Tuxedo generation, and the Plissoft is the scritchiest of the 3, with the most backbone (has the highest glue bump), the Faux Horse is the softest of the bunch to the point where it's probably too soft! It doesn't feel like a brush but more like lathering with a new baby kitten. It's too soft for my brutish sensibilities.
Even though I like using boar brushes none of them perform as well as my Yaqi's and they are very well priced as well, some are around £10 and it'd be difficult to spend more than £20 on one of their synths.
Oumo/DsCosmetic are the new kids on the blocks, I've not used their synths but others have said good things about them, again well priced as well.