What are the disadvantages of using Couchbase's Global Secondary Indexes (GSI)?
The other advantage views have theoretically is that with a large rate of mutations, you'd have a lot of index recalculations.  Views are local to each node, so you don't need to move the data across the physical network to index it.

This all depends on implementation details, the size/types of your documents/indexes, how your system resources are laid out, etc.  Testing should tell you which one works best and note that there is a lot of active development on GSI features such as plasma.