Traffic in Istanbul is a mess. Most of the people live in Anatolian side of the Bosphorus and yet most of the work places are located in the European side. So, daily commuting can be massive in the peak hours as you need to travel across the only two bridges of Istanbul Bosphorus.
Metrobus line is designed to be separated from other vehicles in the traffic and it rides on its own lane ( except for the bridge where the traffic is already faster)
Although it is fast and comfy if you are not seated the long term trip could be exhausting. I believe at least 10 extra busses should be added for the peak hours.
Another problem is that Walking distance at exchange stations ( from subway to metrobus or vice versa) is relatively long ( especially Unalan- Uzuncayir exchange station). This should be decreased as the line is used mostly for decreasing commute time.
As a resident of Atasehir District I have a final statement. Developed as a suburban residential district, Atasehir now has a large white collar population who has long commute time. They mostly have to make cross- bridge trip everyday. Yet, the district is not connected to Metrobus line. Even for the subway you need to go to Kozyatagi station which is not in walking distance. To reach there you need another vehicle either bus or your car, and the number of exchanges you make increases at the very beginning of your journey. This points out a huge transport planning mistake. Plus, the municipality together with the central government’s decision, declares Atasehir as the new financial district of Istanbul (which indirectly means of Turkey. ) Things will probably go worse as the development increases are given before increasing the infrastructure and transport facilities in Turkey.