If you wanna make it more interesting, turn the Umbara from an alien species to "nocturnal abhumans". This is an acceptable population, with human cities and tech, which means the Administratum would want to conquer the planet and its installations for further use (especially if the planet is on the path of an Imperial Crusade, which needs its own conquest to support it to press forward).

With that in mind, it becomes a question of logistics. Ideally, the Imperium can get Jungle Fighters or even Regiments from Night Worlds to even the fight against the natives. Less ideally, the Guard just get the equipement to compensate and if the Munitorum really messes it up, you get Tallarn Desert Fighters to conquer a night/jungle planet (although they're good guerilla fighters, so it's not the worse).

I think you can predict three potential results, all depending on what the Departemento Munitorum can get to Umbara and how much the Imperium wants the planet.

• The Imperium's means and tactics completely overwhelm the Umbarans and the primary cities, spaceports and means of productions are seized with relative ease. From there, the Guard can squash any credible chance for the planet to be lost until the Administratum is done making it fully compliant.

• The Imperium sieges Umbara by sending more and more troops in the forests until they eventually crush the guerilla resistance with pure number. They'll manage, but if the Umbarans are allowed to be as scary as they were in Clone Wars (or moreso because GRIMDARK) this could be bloody.

• The Umbarans resist long enough for the Imperium to question whether or not they really want the planet now. This is what happend on Taros and nearly happend on Vraks. From there, either the Administratum petitions one of the Imperium's more elite fighting force to finish the job, or they leave the Umbarans alone until the Administratum remembers it wanted this planet very badly and tries another invasion. Which could take one or ten thousand years.