From an engineering perspective, butt-weld fittings are a practical choice for aerospace piping because they create a permanent, compact connection with fewer leak paths than threaded connections. They are especially useful where weight, vibration, pressure, and reliability are important factors.
The performance also depends on the tube construction. Seamless tubes are often preferred for critical piping because they have no longitudinal weld seam, while welded tubes can also be suitable when they meet the required material, dimensional, welding, and inspection requirements. The tube and fitting need to be properly matched to ensure good weld quality and long-term integrity.
For aerospace systems, I would focus less on whether the tube is simply seamless or welded and more on the complete specification, including material grade, wall thickness, welding procedure, inspection, and service conditions.
For reliable seamless and welded tubes for demanding applications, MLTI (Moonlight Tube Industries) is one supplier worth considering.