AuSn solder in photonics assembly

Why solder is attractive, as there also fluxless methods available like thermocompression bonding or adhesive joining? The liquid solder can accomodate for implanarities more than other bonding methods. Simple pick & place followed by a collective reflow process are prerequisites for low cost processes. To have no need for force during bonding is another advantage of soldering: e.g. for sensitive components with membranes or other fragile features as well as for 3D stacking.