There'll be lint and moisture accumulation. Depends on whether you consider it to be a problem. May need to open the windows for ventilation when it's running. CO exposure can be a risk with a gas dryer, although not much moreso than running a gas range.

I think a lot depends on your climate and the weather. Potential for condensation on windows and ice accumulating on the windows in winter. Also, mold & mildew, peeling paint, rotting wood. If the roof over the porch is wood & no vapor barrier is installed in the ceiling, water will condense on the underside of the roof and lead to more mold & rot issues. And, as already mentioned, the lint. You'll have lint everywhere and stuck to everything. Of course, if you live somewhere like Yuma, AZ, the moisture won't be a problem or as much a problem. OTOH, you wouldn't want to dump all that extra heat there, either.