I can tell you that last year, we had great bus service from WL. However, 2 weeks ago it was awful for everywhere but MK.
In the am, MK has its own bus. In the pm, it shares with Fort Wilderness. It's a quick stop close to the Hoop de Doo show's theater, though, so not a time killer.
AK and DHS share a bus with Contemporary, and they were both routinely running about 45 minutes apart. This held true at the parks as well- typically 45 minute wait for the next bus. One morning the 8:00 bus came for AK and DHS, and at 9:15 there was a bus for AK but still not DHS in spite of the fact that the person working on transportation had called 3 times for one to come. That 9:15 bus to AK still stopped at Contemporary- I know because I was on it- and it did not arrive at AK until 9:45.
Epcot shares a bus with Fort Wilderness, but at the Outpost. This is a longer ride than where you go for the MK stop, and tends to be more crowded. Again, the bus was running routinely in the 45 minute range. One day at Epcot I saw 3 SSR buses stop next to me to leave the park before one WL bus arrived.
I know not everyone has shared this experience- we certainly did not last year. It may also be corrected with the Copper Creek area being open now. Or, it may just be hit and miss. WL is a beautiful resort, and we enjoyed our time there for both of our trips. However, the bus service was certainly not up to what I would have expected on this last trip.