Yea, there are a lot of doubts as to how complete Metal is in general as of DP1. We have at least 3 months to go for Apple to finish up its work but I have a feeling that Apple is generally happy with how it is now and will release El Cap even with incomplete Metal work to polish it up further in .x.x updates.
Yosemite and iOS 8 didn't feel complete until the .3 updates for both were released.
Apple should extend their 12-month release cycle to 18-month cycle instead, they really need the last 4 months to polish it up.