Yes, you can, depending on the environment. I've done it in prod multiple times. There are a handful of features that aren't 1to1, but they can be worked around in other ways. In most cases, the people I've seen say this either tried Samba pre v4, or had more complicated setups where it may not apply (for example, cross-forest trusts just recently were prod ready in 4.5.1+), or usually just didn't know their way around linux as much.

Is it a replacement for all situations? No. For many though? Yes. Of course there are other options as well, such as FreeIPA, etc.

Samba roadmap: https://wiki.samba.org/index.php/Roadmap