Flux-core wire is a structural wire, so in terms of strength your looking at around 70,000 psi of tensile strength in these welds, you can't really ask for more than that. the biggest disadvantage in your application would be brittleness around your weld (not in it) due to the high heat and quick cooling. I would recommend a post heat and slow cool down for a bike frame. The concern would be that the tubing could break adjacent to the weld, but not actually on the weld. As far as spatter, if the welder is set properly for your material the spatter should be no greater than any other wire feed process. If you are getting spatter that doesn't come off easily with a quick hit you need to reduce your wire speed. Flux core wire is less susceptible to moisture issues than stick rods, so unless your talking about a really wet or humid environment there's no real precautions that you need to take. The issue with stick rods is that the flux coating can loose adherence due to moisture, but this is not the case with flux cored wires. You are correct in that you need no shielding gas with Flux core wire. Just make sure you are not using Dual shield wire, but truly flux core only and you will be fine. The biggest advantage to not needing shielding gas is in outdoor or highly ventilated areas, because the wind will blow away shielding gas causing porosity. Flux core and stick are preferred for environments where you cannot protect the weld zone from air currents. Most high quality bike frames are tig welded, but I cannot see why you couldn't make excellent welds with flux core as long as you use a post heat treatment to relieve the stresses from the weld. Good luck.