Small correction: Using CRC is not optional. Setting the crc flag makes it use CRC32 even for very small packets. The default is to use CRC8 for packets of a few bytes and then upgrade to CRC32 over a small size threshold. This is to minimize the size of otherwise tiny packets.

Also, the ACK implementation depends on the strategy (physical media type), and it is not a single bit but a single byte, and it seems to me to be robust. I am sure the creator, gioblu, has some more comments about this.