Im no expert in these sorts of assessments, but from how is see it:
Okay Skill 1 (70% Damage and 50% chance of Speed down on target.)
Kinda Outdated Skill 2 (Attack and Speed up on one ally for one turn for 2500 View.)
Good Skill 3 (90% Damage on all enemies, remove one debuff on every Ally for 14000/11000 View)
Stat wise he is fast (111 Speed) and average bulkiness (5039 HP at 6*) with passable attack stat for a Non-Attacker (1710 at 6*), and fairly good view generation (2497), effectivbely he is a "Jack of all trades" stat-wise.
His main advantage is the Element since Nether doesn't have much of a dedicated AoE attacker, the only other Nether with AoE attacking is Digram (70%, remove 2 buffs, 14000 View) who has 5 more attack but overall inferior stats, and the lower modifier means Ryekie is effectively better.
He is also a good source AoE Debuff removal and the fact he keeps debuffs at bay while still attacking.
He is less finicky and more View Economical (Cheaper moves, better generation) then his Normal self, but also can't reach the same damage output optimally since Normal Ryekie is has better base attack and can technically improve his 90% AoE with a speed difference, it is just situational.
Comparatively Nessen is a Glass Cannon, with good attack modifiers and moves that Debuffs enemy defense and ups his own attack while still doing heavy damage and is probably in the running of being one of the best units in the game just for being such a heavy damage dealer without any real demands or ifs to do that heavy damage.