Earlier quoted context omitted.
Trams and buses tend to be more unreliable because they use the same streets as cars (sometimes they get extra lanes, but not always). Metros tend to be more reliable in my experience, I can usually trust what it says on the electronic sign (one big exception is Cologne where the metro isn't a real metro and runs on the street half of the time).
However in Cologne, to my big surprise, trams, despite running in the streets and in a real maze of crossings underground, manage to always be on time when I use them. S-Bahns on the other hand, seldom are... Stadtbahns use the "U" sign for U-Bahn when only part of their network is underground; I guess this is to avoid confusing riders.
You might have been lucky. The "metro" in Cologne is notoriously unreliable, to the point that there's even a word "KVB minutes" to explain why sometimes the train will be arriving in 2 minutes for about 10 minutes.