I agree that a properly maintained original Tiger or Anderson timer and buzz coils are reasonably reliable, however they are susceptible to moisture penetration under rainy conditions.
Although there are many purists who would disagree with me, a more reliable ignition system on a Model T is the conversion to a 6 volt Autolite IAY4001 distributor and points in conjunction with a more modern cylindrical coil. This system was original to a 1956 Willy's Jeep CJ-5, and its covered distributor cap virtually eliminates the water issue. Accordingly, although this opinion is sacriledge to most Model T purists, because it eliminates the weakest component of the original buzz coil and timer system, it is arguably more reliable if you are going to drive your Model T any distance under various climactic conditions.
My apologies in advance to the purists out there, I understand that 18 Million Model T ignition systems were produced by the finest engineers of the day, but a minor operational upgrade (available on Dodge Brothers cars of the era) can't be discounted.