Very old thread, but just in case anyone finds this in the future I wanted to post my results. I just converted a 2TB HDD which I use as a Time Machine backup in Big Sur. As OP reported, it appeared to hang on "Reporting pre-conversion statistics" but I checked /var/log/hfs_convert.log and it confirmed there was progress. Spoiler alert: very slow progress. Since it's a time machine drive it basically runs near full capacity all the time, which certainly doesn't help. Here are the head and tail ends of that log file:
Finding inode files for slice 0
1,985,134,215,168 bytes
142,369,710,080 free (7%) before conversion
Creating APFS container
142,071,271,424 bytes free (7%)
[...]
Done converting slice #0
2,595,618 files
699,512 folders
118,571 symlinks
2,030,886 hard link inodes
9,469,386 hard links
18,786,972 inline EAs
3,233 extent-based EAs
0 extent-based EA overflow records
0 crypto state records
0 class 0
0 class A
0 class B
0 class C
0 class D
0 class E
0 class F
Conversion took 278166 seconds
For the record, that's over 77 hours (3.2 days)!