The exiling text ensures this is fit for a high-paid assassin that leaves no chance for reanimation
In Modern, Fetchlands make Descending trivial
Which archetype are you building around
While other options like Traxos, Scourge of Kroog and Karn, Silver Golem might tangentially reward artifacts in your 99 (as if you have much of a choice with a colorless deck), Liberator offers an explicit benefit in being an objectively better Shimmer Myr
You will always need at least one left over to make your commander worth it, but in all likelihood, the amount of cards we're talking about drawing off of things like EarthquakeEarthquake means that one spell will be enough, provided Niv-Mizzet isn't removed in response
This deck will feel at home anywhere - at any table with any other decks and players - for years to come as a valued part of any collection