To explain boarding in Warhammer 40,000, ships are often literal miles apart, with boarding typically occurring in one of three primary ways: being ferried by a small craft like a Thunderhawk, using specialized "boarding" torpedoes (essentially giant drills with Marines inside), or via teleportation.
While ships are enormous, the interiors are relatively tight spaces filled with blind corners and bulkheads. As a result, vehicle and sniper-range combat is extremely rare. Instead, it's essential to optimize for close-quarters combat (CQC), creating pathways and quickly hunkering down when faced with a counter-push.
Devastators, who are equipped with heavy ordnance options, do not perform well in this environment, nor do units with jump packs. Thus, it's best to focus on Terminators and Tactical Marines; however, even Tactical Marines can be too vulnerable in high-intensity boarding actions.
In this environment, doubling down on swords, our primary CQC option, and leveraging Terminators pays significant dividends, especially with Defernse mastery benefiting from encampment.
Thus, marksmen, heavy ordnance, jump packs, and Chaplains represent subpar choices that ultimately weaken the chapter's ability to excel in the role they are designed to potentially dominate.