I made an account just to post this, so bear with me. What can we do to save the Sergeant? As it stands, you are gaining just about nothing by choosing one as a class.
Characteristic Bonus: +5 Fellowship
Starting Aptitudes: General, Defence, Fellowship, Leadership, Perception, Strength, Toughness, Weapon Skill
Starting Skills: Command, Navigate (Surface) or Scholastic Lore (Tactica Imperialis)
Starting Talents: Air of Authority or Iron Discipline, Weapon Training (Chain, Las or Solid Projectile, Low-Tech)
Specialist Equipment: Common Craftsmanship Las Pistol, Common Craftsmanship Chainsword
Accompanying this sidebar is a description stating that Sergeants are the backbone of the Imperial Guard, serving as an example to the rest of the troops (who they are also responsible for training).
The question is… what exactly is a Sergeant good at?
If it's for killing the enemy, we've got many, many classes better at that than the Sergeant. Take your pick: Any class with a combat characteristic aptitude (WS or BS) as well as a combat category aptitude (Offense or Finesse) is going to be better than the Sarge at fighting.
If it's for talking to and/or convincing people, we've got the Priest and the… naturally-charismatic Ratling, because the Sarge doesn't get the Social aptitude.
If it's for knowing things and having skills, we have the Techpriest, the Medic and the Psyker for company. The Sarge doesn't start with the Fieldcraft, Knowledge, or Tech or Intelligence aptitudes.
This puts the Sergeant into a rather precarious position. His number one most useful skill is his sweeping orders. The squad derives a +10 Dodge bonus or a +4 Damage bonus simply for having him there. Other than being the human equivalent to giving the whole squad Best Craftsmanship weapons with the Mono upgrade, what can he do? Evidently, he is supposed to be in charge. Except none of the Leadership talents actually provide any mechanical benefit whatsoever and Sergeants are too low-ranking to derive any benefit from Air of Authority. Just look at what they used to do in Dark Heresy. Comrades are already immune to Fear and Pinning because they don't have to take tests for it! Look at him, he doesn't even have Common Lore (War) or Common Lore (Imperial Guard), and so on. Don't even get me started on the recommended talents for a Sergeant, which suggest dual-wielding to a character that not only doesn't have the aptitudes to pick up those talents, he lacks the aptitudes to even improve his characteristics for the prerequisites.
In comparison, let's look at the Support Specialist (Commissar).
Characteristic Bonus: +5 Fellowship
Starting Aptitudes: General, Agility, Finesse, Leadership, Perception, Fellowship, Weapon Skill, Willpower
Starting Skills: Command or Intimidate, Common Lore (Imperial Guard), Scholastic Lore (Tactica Imperalis)
Starting Talents: Air of Authority, Chem Geld or Unshakeable Faith, Weapon Training (Bolt, Chain, Las or Solid Projectile, Low-Tech)
Specialist Equipment: Good Craftsmanship Bolt Pistol, Good Craftsmanship Chainsword, Commissar's Uniform
Here we have a class who can do the Sergeant's job better than he can (thanks to better aptitudes) while also starting with better training and better equipment. He also technically outranks the Sergeant completely (making it a very bad idea to have both a Sergeant and a Commissar in one squad). The only thing that's missing is the sweeping orders--so go out there and requisition some Best Craftsmanship weapons with the Mono upgrade!