AWS is particularly bad with their performance credit system on RDS... and there's to my knowledge no way to tell MySQL to limit index creation IOPS, which means in the worst case you're stuck with a system swamped under load for days and constantly running into IO starvation, if you forget to scale up your cluster beforehand.
Even if the cluster is scaled to easily take on your normal workload, indexing may prove to be too much for IO burst credits
You can use gh-ost (or most other online schema change tools) to get around that. You'll still have some necessary increase in write load since you are double-writing that table but you can control the pacing of the data backfill (gh-ost gives even more control as it doesn't use triggers to duplicate writes). It's not quite as simple as just running ALTER TABLE on the master though.
Even if the cluster is scaled to easily take on your normal workload, indexing may prove to be too much for IO burst credits