〜もさることながら
X is of course (good / important), but even more so Y
Noun + もさることながら
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もさることながら grants that X is important or impressive, then adds that Y is even more so, or equally so: 味もさることながら、店の雰囲気もいい, "the food is great of course, but the atmosphere is good too".
It's used for evaluations: reviews, praise, analysis of what really matters. The second half often has も or が大切だ: 能力もさることながら、人柄が大切だ, "ability matters, but character matters even more".
It's more elegant than はもちろん (N2) and puts more weight on the second item. さることながら is literally "while that is so".
Don't confuse it with も構わず (N2), "regardless of".
Examples
この店は味もさることながら、雰囲気もいい。
The food here is great of course, but the atmosphere is good too.
彼女は歌もさることながら、ダンスも素晴らしい。
Her singing is great, but her dancing is wonderful too.
能力もさることながら、人柄が大切だ。
Ability matters of course, but character matters even more.
デザインもさることながら、使いやすさが人気の理由だ。
The design is good, but it's the ease of use that makes it popular.
本人の努力もさることながら、家族の支えも大きかった。
His own efforts counted, of course, but his family's support mattered a lot too.