Although the more speeds the higher the complexity, it's not as simple as that. A well-designed 6 speed transmission will last longer than a poorly-designed 4-speed unit. It also matters how well the transmission is matched to the engine, manufacturers have been known to put together some marginal combinations.

One of the most reliable transmissions out there is the 2-speed Powerglide. Although long obsolete for passenger car use the Powerglide is still favored for use in dragsters.

As another example that gears don't relate exactly to reliability, the Toyota/Lexus A750 5 Speed transmission has been made since 2003, and is still made!

It has been used in so many vehicles like the Land Cruiser, Tundra, and 4Runner that get harsh treatment, and it takes it and keeps on going. That is one thing that made me feel better when I bought my Lexus GX470. It is the same transmission that has been recorded as going 500k+ quite often, and even a million miles!

When I am looking to buy a vehicle, I look up the transmission, and see if it has any issues. When you see a type of car that is consistently going 250k+ miles without engine or transmission problems, that tells me it is likely a good one.