Until today, I'm thinking...
Yes, I've had that gripe. I found a workaround last week for artwork changes and was considering posting about it in the next day or two but thought I'd give the updated iTunes app a try. Success, so far.
With an iPhone 6 and iPad Air on VZW's LTE network, an rMBP on iTM, and an ATV all running at the same time, I made about 35-40 changes to albums (artwork, name changes, not compiliations) and genres (name changes) that had "issues" with iTM. My main library resides on a Mini Server, with rips, iTS purchases, and Google Play/XBox Store purchases in that library. My Mini Server, rMPB, and ATV are all on the same network.
Every single change was reflected on my other devices, with the lag being between 3 seconds and a couple of minutes. I'm actually a bit stunned that - for the first time - changes were actually made and that they stuck and are still sticking. Updates in iTunes 12.2 weren't working like this - I'm guessing that Apple finally hired a janitor to unclog that toilet pipe? My iPad and iPhone were seeing the changes in almost real time.
I'm glad to see I'm not alone with this issue. I've been dealing with this since late 2011, when iTunes Match launched.
As you may know, prior to iTunes 12.2, if you inserted new artwork on to an album, the changes would sync across all devices. However, after a certain point, once iTunes Match would automatically update (both on iOS and iTunes), all the artwork would revert to whichever copy was initially uploaded/matched. In fact, if you wanted to instantly reproduce this issue, all you had to do was insert artwork and, right afterwards, update iTunes Match. The artwork would instantly revert in iTunes.
However, the behavior has changed with iTunes 12.2. In iTunes 12.2 (and 12.2.1), as long as the tracks are actually downloaded, it is possible to change artwork and, no matter how many times you refresh iTunes Match, the artwork will stay embedded into the files. Prior to 12.2, this was impossible. Now, though, the changes will stick.
The problem, however, is when albums are not downloaded, and are in the cloud. (Which, at least on iOS, is most of our library, as we're iTunes Match subscribers.) Unfortunately, in this instance, the problem still exists. If I create a new account on my Mac, opened iTunes, turned on iTunes Match and viewed a cloud only version of my iTunes Library, you would start to see the old artwork appear again. Or to give a simpler example: if I simply deleted the downloads of an album in my iTunes library, eventually the artwork will revert. I'm not sure when, because there's no consistency. I might be able to sign in and out of iTunes Match several times and the new copy will stick, but then randomly, it reverts. It's hard to pinpoint when it'll revert, but it eventually will.
I'm glad to see you and
@crizzly bring attention to this, because it's beyond overdue that this bug gets fixed. It's the equivalent of one's contact photos reverting to the initial photo one first used. The problem is that since many people either have no interest in modifying artwork, it's a bug that gets overlooked, so if anyone has access to filing bug reports with Apple, I highly encourage anyone to please file a bug report on this. That's the only way Apple will even notice this, much less fix it.