I have a RF 87ub 55g, and I really like the solid sound and feel of the heavier plate-mounted switches.
The HHKB Pro 2 has uniform 45g Topre switches that are case-mounted. You can do a dome swap from a RF 55g donor board to get heavier switches, but you cannot make the switches plate-mounted like they are in the RF boards.
I go back and forth between the HHKB and RF. My favorite form factor is 60% and my favorite layout is that of the HHKB, but I also prefer the more solid sound and feel of the 55g plate-mounted Topres in my RF.
Sometimes I think that the perfect keyboard would be a HHKB with plate-mounted 55g switches, but this might throw off the unique sound and feel of the HHKB Pro 2.