Some comments i have (not anything serious so far) from a quick first look with an old version of mame (0.142!)...
1. The first thing to notice is that upon startup, the ribbon bar is completely blank!
2. Adding a new folder in a mame db, after loading all files, a "transferring" message appears in the log. I wonder, transferring what, and from where to where?

3. In a mame database, in the left tab where all the game zips are shown, all lines and letters appear normally. In the right tab where the selected rom contents are shown, it seems that for some reason the actual height of each line is smaller than the left tab and that causes the letter "g" to appear cut-of at its bottom.
4. The tooltips of the four filter buttons in the right tab (file contents) are the same with the tooltips on the left tab (i.e. "click to hide complete games" while it should perhaps be "click to hide complete roms").
5. When a rom folder has been selected in the tree, rightclicking on a line in the left tab (roms tab - really, some labels for these tabs would be nice in order to the users to properly describe them), a menu appears. This menu has two entries, "Fix operations" and "open folder in file explorer" which when selecting them the color of the selection is different than the color of other entries (i.e. "rename files"). Also the "fix operations" item's selection does not extend to the full width of the menu. Finally perhaps the "fix operations" should be changed to something more clear, like "fix all roms".
6. Selected a file in the roms (left) tab and then one of its items in the right (contents) tab, then rightclicked on it. When selecting "fix operations" from the rightclick menu, it (i think) started trying to repair all roms in the left tab and not only the selected rom contents as one would expect. At some point, the message "Source and destination path must have identical roots. Move will not work across volumes." Pressed OK but after going to the next rom, the same message appeared again. It may have something to do with the fact that the rom folder was not on drive C:? In any case, this message should not appear in all roms. This has forced me to kill RC from the task manager after seeing the message about 20 times.
7. In step (6), when i tried to stop RC4 from the "Applications" tab in task manager, the confirmation message "Operations running. Cancel and exit?" appeared. When i selected "Yes", it did nothing (because a "Source and destination path..." message dialog was also open). When i pressed OK in the "Source and destination path..." dialog, RC canceled the process but after that, it did not exit as expected. It continued to work normally though, without any obvious issues.
8. Open folder in file explorer: Personally i use another file manager (Total commander) so i would appreciate if there was a way to select the file manager EXE in the options and the menu item was named "open folder in file manager". If nothing has been selected as file manager, the windows file explorer should be used.
That's all so far. Looking forward for the TOSEC xml version support, so i can try my Amstrad collection.
Extra findings...
9. I have a rom that returns a strange message: There was an error on it, so i unpacked it and re-created a new zip (i used 7zip 9.20). This one however, when i put it in the roms folder and refresh it, it returns this error: "Compression level out of range. Parameter name: level0 to 5". Even if i compress one of the roms inside it to a new file and put it in the rom folder, it produces the same error. I also compressed them with my file manager (Total Commander) which uses a different set of DLLs for zipping but the result was the same.
UPDATE: This started since i went to file/options and changed the zip compression level to "max". After that (and the rezipping), the issue occurred. Returning the slider to its default value eliminated the problem.
10. Typo's: "Create a new database from an emulator": the destination file's description is wrong. It should be the same as in "Create a new database from a datafile". Also, in mame filtering: "You can filters roms..."