Agreed. Sergei Lyapunov is highly under rated. His Sonata in F Minor is beautiful, as is his etudes.
Anyway, other composers that are unappreciated are people like Julius Reubke. An eminent pupil of Liszt. In fact Liszt once said:
'Truly no one could feel more deeply the loss which Art has suffered in your Julius, than the one who has followed with admiring sympathy his noble, constant, and successful strivings in these latter years, and who will ever bear his friendship faithfully in mind'
Additionally, I would say that Alkan is slightly underappreciated too. His etudes are amazing and he conveys a distinct style. I love him (as a composer, lol).