for FEOBOS is 8-10 move length really a problem!?
Short explanation:
Often ECO codes formed after 1 or 2 moves. If I create now an opening book undo 8-10 moves, to many of good lines for such openings are in the book and opening will be to often on the board. Much other ECO formed formed after 7 moves ... and not enough lines are in the book (other example).
So for FEOBOS:
3 moves after ECO codes formed. Often for ECO codes, formed after one move ... I am working all the time with 500 databases for opening book optimation.
That is indeed a problem because with book prioritys by GUIs lines with more moves will be play more often.
Example:
B01 is formed after 1. ... d5
B01 is normaly in GM theory from 500 ECO codes a TOP-50 code (around on place 30-40). If I added now 8-10 moves for B01 ... B01 is to often on the board with random book options by chess GUIs.
Most book creators forget it.
FEOBOS 20.1 is here very strong.
If lesser balanced positions (for each code 3 moves after ECO Code formed) ECO codes will be play rarely. And if you now compare with GM theory ... most popular lines will be comparable to FEOBOS.
To 100% we can't create the perfect book but in the near of 100% is possible.

Best
Frank
PS: With FEOBOS and 10 engines analysing each of the lines in book ... the topic is over for me. I have the "perfect" book for eng-eng testing ... created in team work by 10 strong engines with our Excel file. Have a look on FEOBOS Detail page ... all databases in *.pgn, *.epd the Excel ... 33.009 lines sorted with a ranking system in *.pgn file are online. Each of the 33.009 have an own ranking (important if you like to create a test-set).