First, if you have an older tractor / car an LED turn bulb will more than likely cause problems. As stated correctly, an old style flasher uses heat from the current draw of the light bulb to bend a piece of metal in and out of contact (heating and cooling). Without a resistor, a plain LED bulb will not work, either flashing quickly or not at all. The resistor LED is no guarantee of fix. On newer vehicles the issue can still remain due to how the electronics are programmed. Many electronic flashers are "smart" looking for a burned out bulb. Due to the low pull of an LED the system will indicate broken bulb and again act hinky.
Finally, the problem with LED lights in an automotive application (or heck many applications) is design. On paper the led will produce much more lumens than an incandescent, but because of the bulb not being designed correctly for using the reflective inside of your flasher housing, the bulb will appear dimmer. IMO the best bulbs for this application are ones that are flat, with all LED's facing forward. As higher wattage LED's become more common, and the supporting circuits beefier, this issue of not enough lumens hitting the lense will change.
Finally, it should be noted that small tractors (excuse the offense here) but ones that run on a riding lawn mower engine (24HP Honda or Kohler for example) tend to burn out LED Bulbs due to the alternator design.
