Some of the details here [In Couchbase 7.0.0 Beta, indexes may stuck in building 100% state - #19 by zoltan.zvara]
Another issue we found is that after node restart, some indexes stuck in Error state and must be deleted then recreated to be fixed.
With 100s of indexes currently our single 4-node Couchbase 7.0.0 beta CE cluster, it is very hard to move along without index issues when a new application starts up or when a node goes down and comes up.