The LuxeErgo 3400 is described as firm. Is it too hard for a side sleeper?

It depends on your weight. The LuxeErgo is a firm 31cm hybrid, but the top layers are plusher than the firmness label suggests, which is why it scored 9 for comfort as well as 9 for support in our testing. A side sleeper over about 12 stone will sink into the comfort layer enough to be cushioned at the shoulder and hip. A lighter side sleeper will sit on top of it and may find it hard; for them the Luxe Velour Hybrid, a medium, is the better choice from the same maker. Luxe gives 100 nights to decide, so you can try it.