Roland FA, Juno DS MODX
Played them all and there is not a seamless transition close yes but not seamless.
As I said, there are different implementations.
Kronos is fully seamless, as long as you only need to hold a sound through a single transition. (Before someone says that's all anyone needs, it can indeed be useful to keep a chord droning with one sound, while you change the different sounds you play above it, for example.)
MODX is as seamless as the Kronos, but only if your sounds consist of no more than four split/layered parts, as opposed to up to 16 parts on the Kronos. This is complicated slightly by the fact that sometimes a single Yamaha sound can take up more than one part, but that can also be easily avoided once you are aware of it. If you stick with combining individual sounds that are only one part each (which is the only kind of sound the Kronos has), then it will be as seamless as Kronos, albeit only for split/layered combinations of up to 4 sounds. (It's up to 8 sounds on the Montage.)
Juno DS is not fully seamless in that there will usually be a glitch in the held sound when you call up the new patch, but you can minimize the issue by managing the effects of the sounds you want to switch between. (Also, in single patch mode, switching from a polyphonic sound to a monophonic sound will cut off the poly sound, though that's not an issue in the multi-sound Performance mode.) OTOH, it will hold a sound through multiple sound transitions, which the Kronos and MODX can't do.
ETA: Juno DS also lets you use its pads to switch among the sounds WITHIN a 16-sound Performance entirely seamlessly.
I'm not as familiar with the Roland FA, but IIRC, switching between individual sounds ("tones") is seamless, except for the SuperNATURAL Organ sounds. But for switching among multiple sounds at once, you can only do that within 16-sound definable Studio Sets. But at least those switches are totally glitch free, and I think they hold through multiple transitions (maybe someone can confrim that?). As long as a given song doesn't require more than 16 sounds in various combinations, you can seamlessly switch well with this approach. The downside is that you do have to set up your 16-sound songs ahead of time. You can't arbitrarily switch seamlessly from any sound combination to any other sound combination on the fly... Kronos and MODX would be better for that, but then you trade off being able to hold a sound through multiple transitions. Nothing's perfect...
In current boards, almost everything has some implementation of seamless switching. The Korg SV2 added it (the SV1 did not have it), the Nord Electro 6 and Stage 3 added it (the Electro 5 and Stage 2/2EX did not have it). Though besides the Kross, I did remember that the Numa Compact 2X does not have it. (Well, it works in some cases, but pretty limited... the earlier Numa Compact 2 actually did a little better in that regard!) I'm not sure about the Krome? From what I know of its architecture, it would not have seamless switching for Combis, but might allow you to switch from one Program to another without cutting out...? Kurzweils and Casios have pretty much always been good about offering at least some amount of seamless sound switching.