I've been using temporal.io, which afaik is developed by some of the people who built SWF (and Cadence at Uber), and I think it's great. I'll probably use it for a ton of stuff in the future.
Temporal is even better as it replaces Thrift with gRPC and adds tons of improvements.
Would be interested to know what kinds of things you're using it for/what scale/which language client?
About two years ago I built a custom workflow engine in python where I had a few primitives (filesystem, database access, browser (puppeteer scripts), etc), and would define a workflow using those primitives using a YAML DSL.
In a couple weeks I ported the whole thing to Temporal (Java client), where each of those primitive groups is a different activity, which runs in a separate worker. I have a browser worker, which runs in the only servers that have internet access, a filesystem worker, which is on a Windows machine serving as an entrypoint to the company's windows filesystem, a database worker, etc.
This is all very scalable, and I can run as many of each worker and the engine itself as I like.
Currently the scale is not too big though. I launch a few "main" workflows in certain days of the month that will then spawn up to 4 thousand child workflows which run simultaneously with no problem.
This discussion has more on Airflow vs Cadence: https://news.ycombinator.com/item?id=19732447
To understand why Cadence is so useful, you have to go a bit beyond the traditional ETL scheduling usages IMO.