Grendel has about ~60% of Inaros' effective durability (EHP), but also has a shield, allowing him to make use of shield-gating, which is a massive benefit. His shield is also extremely low, making it better at shield-gating.
Add on top of that [Grendel]'s ability to buff Toxin damage and you've got very little reason to keep using [Inaros] for anything other than Hijack missions.
Subsume Roar and you've got +Toxin damage and +Faction damage (Roar works like Faction damage) buffs benefiting you and your entire team.
Adding Umbral polarities to the build will allow fitting Primed Feet in Exilus and more expensive mods than [Augur Secrets] and Message, but this is more than good enough for 90% of content.
Missing Primed Feet(it's a login reward) or have Umbral polarity? Fill the slot with [Gladiator Resolve], [Adaptation] or [Narrow Minded](and swap [Augur Message] for [Stretch]), depending on whether you wanna increase durability, lazy durability or ability usability.
Can't wait for Grongol Prime so we can get a more comfortable Energy pool :D
(and more incentive to add Umbral polarity)