The Bossar is pretty great, but you have to remember that all the long-range automatic fire weapons spend most of the game playing catch-up to the Heavy Machine Gun and Heavy Gauss in terms of raw firepower delivered to target. The Bossar only just reaches HMG/PS rounds for damage, but makes up for it by being useful on the front lines as well. It still can't beat a Heavy Gauss blast at close range, and you can loot those within the first year.
On the assault rifle front, it eventually has to compete against the likes of the Battle Laser (similar weight and firepower, but ignores some armor) and the XG Rifle (nuch lighter weight, damage scaling with accuracy). The Bossar is there to make the equipping phase quicker - there's a better weapon for pretty much every role, but eh, I feel like being lazy and just using the Bossar for everything.
If you want to feel the need to diversify, try things like the Vulcan RFG, Tornado Rocket Mortar, and XG weapons.