The Stellar Guard ground forces believe in quick strikes and mobile warfare. The Stellar Imperium does quite a bit of conquering, and the Guard has gotten very good at it. With their mobile forces, they are passable at anti-guerilla operations, but they hate static defense. The regiments and above are structured so that unit can maintain some form of limited offensive operations, even if these operations are only spoiling attacks.
This aggressive posture makes up for the fact that Guard forces are usually outnumbered, and often outnumbered heavily. Static defenses are often useless in such situations, since the enemy will simply roll over the positions. Instead, the Guard hits and runs, using mobility and superior technology to stay ahead of their opponents. Since the Guard usually has aerospace superiority, reinforcements, resupply and/or evacuation are just a shuttle ride away. This margin of security allows Guard officers to take chances that other commanders can't.
The biggest weakness the Guard has is that this strategy demands a lot from commanders. If they aren't up to the challenges that mobile warfare throws at them, then they don't have numbers to fall back on. Good commanders can excel; bad ones can get destroyed.