As a person who works with a lot of stereo sources, I agree with this.
In general I prefer desktop effect boxes or rack effects for synths.
That said, I have a bunch of those old Line-6 ToneCore modules and three of the stereo dock pedals and they work pretty well with synths. They were discontinued but if you are patient you can find the these pedals for <$50. My favorites are the Verbzilla, Echo Park and Liquid Flanger. The Ottofilter and UberMetal are also pretty cool.