If you get a quality tig welder aluminum isn't too bad. It's definitely more difficult than steel but I taught myself to weld AL without too much trouble. Practice on some scrap for sure before your bike - it'll be easy to blow a hole in thin bike tube.
The biggest challenge I've had in welding aluminum as a hobbyist is that I rarely know what aluminum alloy I'm working with. Most things don't say what type so we're left guessing what filler is appropriate. If you use the wrong filler it could be prone to cracking again in the future.
Also for thicker aluminum preheating is very important. The aluminum transfers heat away very quickly so you get cold lumpy welds if you don't have both parts pretty hot at the start.
The biggest challenge I've had in welding aluminum as a hobbyist is that I rarely know what aluminum alloy I'm working with. Most things don't say what type so we're left guessing what filler is appropriate. If you use the wrong filler it could be prone to cracking again in the future.
Also for thicker aluminum preheating is very important. The aluminum transfers heat away very quickly so you get cold lumpy welds if you don't have both parts pretty hot at the start.