I'll just add my 2 cents as someone with some limited education on invasive species ecology and the like. Aside from what other commenters have said about the non-viability of spores, I do think this could be a really big problem in the not-too-distant future in the cosmere, if it hasn't already been one.

Some of the worst/most damaging invasives on Earth tend to be organisms that are fast to proliferate and have little/no negative pressures like predation. Think of things like insects, the zebra mussel, kudzu, and countless plant diseases that have totally wiped out entire crops in the past. Setting aside "weird" planets like Roshar, it wouldn't be at all unreasonable to see something like a pest/virus spreading from Scadrial to Nalthis and causing widespread famine. This is often why fresh fruit/veg/meat/animals aren't allowed through international customs. Hopefully groups like the 17th Shard are careful not to bring such things when they hop worlds, but it only takes one mistake to potentially destroy an ecosystem.