First my Shyvana, then Sejuani, Malphite is next.

Shyvana was fine before she received her first mini-rework if you knew how to play around her strengths and weakness, but because of unnecessary buffs became fotm ban and subsequently nerfed. While she was still permabanned status I found joy in another underused champion,
Sejuani, who is very well known now after significant changes to her kit and items and is getting hit now. Again a champion I actually spent time to understood and see hidden potential in was buffed only to be nerfed again. Again the champion I played became permaban during the process forcing me to find Malphite. While not as obscure as the other two were at before their changes he is still underplayed despite his strong scaling and teamfighting making him a potent pick if you understand he's a champion that might not win lane, in fact that shouldn't be you're goal at all. Much like Shen he's more potent at teleporting across the map, sacrificing lane presence to net his team kills and dragons immediately when he unlocks his ult. But because people like strong laners who can snowball themselves and split push he's not popular leading to upcoming changes to his W next patch:

Malphite Brutal Strikes (W): Passive Changed: Malphite gains 10/15/20/25/30% additional armor from [Basic Attacks gain 30/38/46/54/62% splash damage] Active Changed: Basic attacks deal an addional 25/40/55/70/85 (+15%AP) physical damamge to the target and nearby units from [Malphite gains additional armor and attack damage]

[CLARITY] Brutal strikes now displays a visual effect that indicates the effective area [CLARITY] Basic attaks made while under the effects of Brutal Strikes will no use Malphite's critical strike animation

The buffs likely will draw attention to him as they're directed at helping him trade better early game making him more in line with other toplane picks while keeping all the other insane strengths (namely his insane ult) to the table and likely would mean nerfs to his ult later on and end up homogenizing yet another champion.

Tl;dr There are some champions who genuinely need help, but not every underplayed champ is underpowered. Some aren't played well by the average league player or are off-meta but strong if the meta shifts. These champions don't need buffs but rather they need time instead and such buffs to them only make them overpowered when the time people do learn them or the meta shift comes.