Dielectric unions are garbage, there's a little plastic sleeve inside and that's what always leaks. You can try to tighten it like you would anything else (the outside flange portion) but it would only be a matter of time, I would just replace it with brass.
I agree to an extent, but that plastic sleeve you're speaking of is only to keep dissimilar metals from coming in contact, nothing else.
The leak point of that connection is the rubber washer where that brass sweat fitting comes in contact with the rubber and the top of that dielectric union.
I use these connections exclusively even though galvanized clogs up over time. I go an extra step and cut small sections of dip tubes, flange side up and insert through the opening of the female dielectric that brings one solid plastic port from dielectric nipple to top of union.
That distance is 3/8" where those galvanized threads are exposed. Sometimes less if you can hard crank those dielectrics down.
I'd love to have the capital to deal with china, build dielectrics that are brass with teflon washers that provide a better longevity. Good brass never goes bad over time.
Realistic point of view would be the plastic inserts I've been installing for years inside that union to keep the galvanized from closing up the connection.
Too often I've seen 3/4" FIP's to dielectric nipples leak, contributory to the galvanic corrosion that starts from the go when those two metals come together. There is a slight area (first row of threads) that are in contact with water and copper and that's all that is needed to make that situation start destroying the galvanized.
Code in my area requires an interruption of continuity through that tank. I don't care if water works as the "connection"...I'm talking about copper to steel or galvanized.
Removing those factory installed nipples voids the warranty on most tanks, some you cannot even get to them to remove them, some have heat traps.
Something goes wrong with that heater and a factory rep comes out to cover the warranty on a problem, you've just given them good reason to pick and nitpick and not cover the problem.
A brass 3/4" by 3/4" DUF (Dielectric Union Female cc sweat by ips) would be a nice addition to the plumbing profession.
A product you can reuse from water heater to water heater, short of new gaskets.
That is the reason why you don't see them; can't make money if it is a lifetime connection.