In that case if I were you I would investigate with flac files first, as they have the best and most consistent tagging structure. Well documented so all applications should produce the same result.
As you mentioned Comments aren't shared correctly between MC and Metadatics, it looking more like an issue with Metadatics not saving tags correctly. Investigate what Metadatics actually writes into the files. Using a HEX editor to look directly is the best way to do this, as most tagging applications manipulate the data they display, and don't show the raw tag entries.
I use "Tiny Hexer" version 1.8.1.6 because it is small, fast and free. Google will find it, but be careful where you download it from, as it is everywhere and some of the sources would definitely have put malware or viruses in the download. Any Hex editor should work though.
The Special (or composite) Characters are another issue. There are a lot of threads discussing that now. I haven't seen a resolution yet, but the cause is the Code Page or Character Set used in creating the Special Characters.
EDIT:
Zybex has got in before me. I suggested testing first with flac files because they use Vorbis, and not ID3 tags.
If Metadatics is using ID3v1 tags it is way behind the times. I have a few MP3 files that still have ID3v1 tags in them, and the [Genre] tag, as shown in the MC Tag Dump, is shown in the format "Rock (17)", for example. The same files has an ID3v2 tag of "TCON (Genre): Rock", as shown in the MC Tag Dump. I suspect MC understands ID3v1 correctly.
As above, look in the MC Tag Dump.