2 issues...

You will face a monster2 samplefolder problem message...
mame 81u8u9 lists monster2 as a clone of monsterb but the driver didn't include the flag to use the sampleset of the parent.....cmpro will ask you to use monsterb which is correct. Should be fixed in the next mame release.

the area51 issue.

in u8u9 we got a new area51.chd file. the old one became area51a.chd. That's ok....but:
the clone a51r3k uses the name of the new file but the sha1/md5 values of the old one....great stuff....and you'll end up with a chd-clone-in-clone issue...

Mamedevs know about that issue...and will probably fix it till next mame release.

Roman Scherzer
ClrMamePro