They mean very similar things since they both specify the date. "as of" can be used to clarify that on that date the qualification was granted and was in effect, but it isn't making a statement that the qualification hasn't been revoked for some reason. To be specific, that engineer's qualification could have been revoked in August 2019 for misconduct of some kind. Then you could say it was effective as of June 21, 2019 but was later revoked. Anything that engineer did between those times may be legal, but after it was revoked she couldn't claim to be an engineer anymore. Also the qualification board's decision could have been dated June 21, 2019, but the qualification may be effective as of July 1, 2019. So they passed their decision, but the person can't use the qualification until July 1. Here's an example of "as of" where it goes the other way. Laws can be in effect before the time they were passed. For example a tax law passed on March 31, 2019 can be effective as of January 1, 2018. That means the effects of the law apply to every transaction in 2018 and will affect any tax filing after that date.

Thanks a lot, Ethan!

There aren't really any differences between "as of" and "dated". We don't say: "as of dated (date)" as this is incorrect. "dated as of (date)" is correct but redundant. "as of" can be shortened to "of" in this context and placement. "dated" / "as of" / "of" / “dated as of" + (date) are all correct in this context.