I think most of the relaxing herbs have a potency and/or saturation point problem, when being used as an alternative for cannabis. I have tried about 5-10 different ones, both extracts and dried-but-further-unprocessed plant material. I have tried on empty stomach in the morning while sober, I have tried while high on thc and other psychedelics.
Most drugs seem to have a saturation point. The best example to explain this, that I can think of now, is the fact that mushrooms and LSD seem to reach a point of maximum intensity and otherworldlyness that ayahuasca can easily add some more to. Iboga goes even further out of the realms of ordinary consciousness and cannot be compared to the intensity ranges of the classic psychedelics. So at a certain dose, a drug reaches a saturation point and the intensity cannot be any higher. Just like taking more than 500 mcg of LSD does not give stronger effects in most people (I refer too LSD Psychotherapy by S. Grof, who talks about this saturation point as well).
In my mind the saturation point of many cannabis alternatives is a lot lower than thc, so this means by nature they already have a limit as to how high they can make you. But I haven't performed enough experiments or reading up to be entirely sure about this point, so criticism is more than welcome.
Add up to this the potency problem: You need to administer large amounts of plant material because of the low potency of the plants. Usually it is smoked. Most of the smoke you inhale is inactive, so it takes many draws, puffs and lungfulls to actually start feeling something. You can overcome this by using extracts. I have tried extracts of dagga and kratom and they were quite nice.
Anyway, I think high doses of vaporized cannabis or pure-smoked hash or cannabis is a lot more like very low doses of classic psychedelics than any of the catmint, passionflower, damiana, mulungu, mugwort, etc. herbs. Combinations of low doses of psychedelics with relaxing herbs like kratom might be interesting to research.
What also comes quite close to the relaxing properties of cannabis, in my experience, is ingesting 3-5 grams of kratom resin extract. Then again, some people report itching or increased energy levels on similar doses, so it's still a matter of trial and error in this respect.
There is, btw, a cannabis church, so you can, according to what they claim, legally use cannabis all over the world for spiritual, religious purposes when you become a member (see
http://www.thc-ministry.org/). I talked with the guy who runs the Amsterdam office and he says they've won several cases in Holland and the US, preventing people from ending up with severe penalties for having a couple of joints (US) or for having 80 plants (Holland).