FictionSpren

Yes a spren probably can not travel but since nahel bond exists in the spiritual DNA and therefore in the spiritual realm, the bond exists so a Radiant will be able to use both of his surges and all the other things like healing etc will happen. 
However, it might take too long to summon shards on other planet. 

The Spren are bound to the planet of Roshar or maybe the greater Rosharan system. It might be possible to hack a way around it but it's bound to be difficult. 

Metal however is not.

It's like this i think. U don't find Spren everywhere so now the Spren identify only roshar as inhabitable . Hence , even tho u can find honor and cultivation everywhere , it will be hard to take them offworld .

To thr Spren it's Kinda like trying to make you breathe underwater. 

Then there's the fact that Spren predate the Shattering. Adonalsium might have sculpted them itself and for some reason he made them endemic only to the three inner planets . The arrival of the shards gave them a major boost but the foundation remains the same , sculpted and limited by adonalsium. So I guess that's why Spren can't go offworld easily and yes , @The traveller the bond is Spiritual but the Spren exist on all three realms . On two of those Spren are bound to Roshar . Idk if you can surgebind with only a bond , with the Spren being located light-years away. 

Metal on the other hand is not endemic to any planet , it's not sentiant or sapiant either , so it can't go boxing itself to only one world .

So that's the metallic arts are universal. Breaths are universal for similar reasons I think. Breaths were created by Egdli and not adonalsium and probably fairly recently too. The breaths aren't exactly sentient either. They make things sentiant but still require commands so I guess u could call them robot level sentiant.

Sel's system is bound to the Dor.  Hmmm , wait I understand on Sel devotion and domination are merged but what about the rest of the Cosmere ??? Can someone access old SEL based magic systems offworld. I'm not talking about the the Ire. I'm pretty sure Elantrianic magic is bound to the Dor.  Is there someone offworld, alive right now  who could remember or atleast rediscover the magic system . Wait is thst what hoid is trying to do with the moon scepter ?? Access pre dor selish magic systems ?? It wouldn't work on Sel but offworld ???

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So I guess that's why Spren can't go offworld easily and yes , @The traveller the bond is Spiritual but the Spren exist on all three realms . On two of those Spren are bound to Roshar . Idk if you can surgebind with only a bond , with the Spren being located light-years away. 

I still think that since nahel bond changes the spiritual DNA of a radiant and the radiant is broken his soul is split wide open which then the bond the spren fills with investiture, essentially I believe spren and radiant become one spiritual entity, 

mans because the nahel bond exists in spiritual realm and impacts the spiritual DNA so even if spren stays back on Roshar, the bond is not broken and surgebinding would still be possible.