Not all aluminum tandems are quite stiff and unforgiving for all riders. It's true that the old aluminum Cannondales, with their straight-gauge and relatively thick-walled 6061-T6 tubes, have a fairly harsh ride in comparison with (probably) all other tandems, including all other aluminum tandems. But other aluminum tandems, made from the thinner 7000-series aluminum, often butted and with ovalized bottom tubes, have a much smoother ride. The "ZR-9000" aluminum tubes used by Trek and Burley weigh scarcely more than the 7000-series tubes, which suggests they also would have a smooth ride.
Conclusions about the ride qualities of tandems can be dependent upon the physical characteristics of the riders. A very small and light team will find the ride of a tandem to be much stiffer than a stronger and larger team on the very same bike.
Further, production tandems and the tubesets from which they are made are designed to compete in the marketplace and are intended to make a particular weight and perform best for the average buyer. This requirement imposes some disadvantages for some teams.
1) Making the tandem smaller (shorter lengthwise) allows the tandem to make a commercially competitive weight with components of moderate cost, but also ensures that good fit cannot be had for stokers any taller than 5'2" on production tandems, and even they will appreciate a little more space, which is only available by paying the premium for a custom tandem.
2) Steel is a relatively dense material, and the smaller-diameter tubes (compared with aluminum) of commercial tubesets are not well suited to large, powerful teams unless additional tubes are used, which runs the weight up. The tandems built from steel may have excessive flexibility for large teams, which can lead to poor handling and poor power transmission for the tandem captain. If the bottom tube does not have sufficient lateral rigidity, which can be the case with the smaller tubes of a steel tubeset, then some of the power of the captain will be wasted.
Absolute statements about materials often are not correct.