Trying to find a reliable source of information about health issues on the web isn't easy (unless the NHS website has relevant information or there's a well-respected charity dealing with specific issues).
However the American WebMD site is usually worth a look:
http://www.webmd.com/vitamins-supplements/ingredientmono-539-labrador%20tea.aspx
Most of what appears there simply says "There's not enough information available" but the 'Side Effects' tab is worth clicking on, as it explains why (as you suggest in your post) it should be taken sparingly.