There is a potential for loss of data if the master fails before any newly written data can be replicated to other nodes.
Writes are replayed to the read replicas. If there are a lot of writes, the read replicas can get bogged down with replaying writes and can't do as many reads.
The more read slaves, the more you have to replicate, which leads to greater replication lag.
On some systems, writing to the master can spawn multiple threads to write in parallel, whereas read replicas only support writing sequentially with a single thread.
Replication adds more hardware and additional complexity.