I'm having as much luck getting on the NetBSD mailing list as the DragonFly one - i.e. I'm not - so I'll post on here in the hope that someone on the team is listening!:
I had problems trying to install 9.3 on a 280R, OBP 4.16.4 on a 300GB HDD.
I initially ran sysinst with the default settings (i.e. deleting all partitions and letting NetBSD re-create them), and at reboot I got an 'Inode not directory' message. I found a previous item in the mailing lists c.2015 saying this was due to too large a '/' partition. I notice in the up-to-date install instructions it mentions this for Ultra 1 and Ultra 2 machines being limited to 4GB.
I went through the install procedure again, this time trying to resize the partitions/filesystems, but there was some weirdness going on. I didn't notice at first, but the partitioner would initially set a size correctly (like 4096MB for '/'), but then when I went to adjust /usr it would change the size on / as well (to something seemingly random, in my case 2896MB?). Then it was complaining there wasn't enough space to change /var, even though there was lots!
I went round in circles for ages and tried different approaches in sysinst until it finally gave me what I wanted (by luck as much as anything else), and managed to get a bootable system - but it was painful.
I'll also state that on a previous attempt following an OpenBSD install, the NetBSD installer seemed happy with OBSD's partition layout and tried to install its files on top, which needless to say ended in disaster.
So to complete this waffle (and tl;dr) it looks like the partitioning process in the new sysinst needs some work, at least on sparc64.