The difference between druids and shamans is political and doesn’t have anything to do with different types of beliefs or practices. A druid is simply a shaman with the power to make decisions affecting other people, like a tribe, a clan or a nation. In short druids are rulers and shamans advisors.
In the modern world of today nations under druidic rule are uncommon. As nations grew druids ruling a tribe or clan gradually gave way to lords and kings governing nations. These in turn gave way to modern constitutions in their various forms – at least in most cases.
Shamans are still very common though; employed as advisors by both governments and private interests they give advice on matters affecting their designated areas. The advice shaman provides is based on how the spirit of the land will be affected by the proposed matter. If there’s a change to be made in some way, how will the spirit react? Will the change affect the spirit in a positive or negative way? How will the spirit of the land react if it reacts at all? The shaman consulted presents their assessment and it is taken into account before the decision is finally made.
These areas designate shamans are usually on the local scale; cities, regions and even provinces are the most common ones. A single shaman advising on a larger area, the size of nations and above is uncommon but not unheard of. More common is that groups of shamans come together to analyze a decision and the effects it would have on a nation and then jointly present their advice.
It is this dependency on location that is the main weakness of druidic rule. Druids and shamans interact with the spirits of the land in the same way and they require the same familiarity with the spirit to do it. Maintaining familiarity with the spirit of a nation is difficult and time consuming. Depending on the size of the nation it may also involve significant amounts of travelling. Topping this off with the burden of ruling a people and druidic rule soon becomes unwieldy and inefficient.
Still, nations under druidic rule still exist. They’re not many, they’re always small and they’re often prosperous. While the nations are not necessarily economically rich or technically advanced their respective peoples are often both happier and healthier than in many large modern nations.
Naturally there are exceptions to this rule but overall it is quite clear that druidic rule is successful in the smaller scale, if not in the larger. This is why shamanistic advisors are so common and why they are used more or less all across the world. Their advice may not always be followed but it’s generally both heard and valued.