Here in Berlin there are an enormous amount of kebab places (thousands). Basically, most places are embedded in neighborhoods and have a crowd of locals that go there. The quality varies wildly as a lot of these places are obviously aimed at the money laundering business (cash only) rather than serving food. Poor hygiene, indifferent service, etc. Many of these places are completely pointless from a business point of view. Other than the money laundering thing. They are easy to recognize since they look kind of old and dirty and there are usually not a lot of customers. If it's lunch/dinner time and nobody is eating, maybe skip it.
And then there are some really outstanding places with proper char coal grills, staff that knows what they are doing and are friendly and laser focused on customer service. These places are wonderful and generally good value. Some of these marinade their own meat as well and they venture beyond just serving the standard kebabs. If you find yourself in Berlin, worth getting some advice on where to go in your area. There are some amazing kebab restaurants and most of those places have been around for ages.
And then there's a third category of places that you find in the usual late night and travel hot spots targeting travelers or people that are drunk that will probably not be regulars. Competition for this audience is fierce and there are some good choices here that people will take detours for to get to. Many of these places are well run and optimize for throughput and consistency (but not necessarily high quality).