Since I don't have mine yet, this would probably be the thread to say this in. There are a few things I know of that I see as potential cons:
- ABS spacebar
- Too light
- Feet not grippy enough
Actually, I thought there was more than that. Someone dispel my concerns 
That it's light is a plus if you carry it with you. I guess the Pro2 is the same as the JP when it comes to grippyness (which is terrible) but that is easy to fix with some cheap rubber pads
HHKB Pro JP has PBT spacebar but is smaller in aesthetics. Not sure on the plastic used on Pro2 but Pro JP it is ABS. Pro2 has a "bit of a" con and that is with USB hub not able to
power up certain equipment. Also one other person has reported issues with it
not being compatible with iPad or something.
I don't have HHKB Pro2 so I wouldn't be able to test either of the cases however.
HHKB, in fact Topre are also rated at
30 million key presses or more (warning link is in Japanese) as opposed to Cherry MX (for instance) whose are rated at 50 million key presses. Topre key caps in general are not as common as Cherry MX key caps but replacement key caps are usually PBT based plastic with dye-sublimation (to prevent both yellowing and/or legends wearing easily). Again Pro2 spacebar is only in ABS unlike Pro JP which is PBT. Topre switches are designed distinctively different from Cherry MX. It maybe easy for one to buy Cherry MX switches and replace them with some tools but the same thing cannot be done with Topre switches. The rubber dome sits over the conical spring and all these are sitting on a single PCB. Also one cannot buy lighter switches as a single unit/piece.
They must buy the entire keyboard to harvest the rubber domes for instance out of that other keyboard for use with HHKB.
HHKB Pro in Japan, apparently
is was
cheaper for one to buy the HHKB Pro JP as opposed to HHKB Pro2. (link again is in Japanese). HHKB Pro Type-S series are an exception here.
HHKB Pro2 lacks a few keys like right control, dedicated arrow keys and is more orientated as ANSI layout. HHKB Pro JP virtually follows the JIS layout but also incorporates dedicated arrow keys. However at the expense of all that the DIP switch settings on both the HHKB Pro2 and HHKB Pro JP are
different. (link again in Japanese)
HHKB like Realforce, Leopold, etc generally has their own unique set of key cap dimensions. On most keys like
alphanumerical key caps it is easily replaceable/interchangeable between most Topre based keyboard. The modifier keys are not as easily interchangeable. HHKB Pro2 may have closer similarities with Realforce spare key cap range but you won't be able to find a
red left control key for instance.
One more point I almost forgot to add, HHKB in particular has the "slider housing" molded as part of the keyboard case. So that makes designing a custom case a little more challenging for instance.
