They should use a digital flasher unit, but if they burned out that fast I doubt that was the issue. Most likely it was the quality of the LED bulbs you bought, but you might want to check the voltage that you're getting at the tail lights. Outside of having too much voltage, or a lousy ground, it's just the quality of the bulbs you bought.
These are supposedly the best. Brighter, longer lasting, less voltage, DOT legal.