Commanders of the empire warhammer 8th edition empire overview
In the game
The commanders of the empire fall in to two groups, generals of the empire which are a lord choice and captains of the empire, who are a hero choice. Both return in 8th edition and are very similar to there 7th edition incarnations though with some important changes. The biggest boost they have gotten in 8th edition is the "hold the line" special rule this allows a unit the character is accompanying to take break tests on 3 dice and pick the best 2. Only mediocre stat wise (for heroes and lords) the cheap cost and massive choice of equipment and mounts still make them a viable choice and the "hold the line" special rule only enforces this.
The general has access to 3 mount choices the imperial griffon, the imperial pegasus and the more mundane warhorse, the captain has the choice of imperial pegasus or warhorse. Both of course can also choose to enter battle on foot, though generally the increased save from being mounted potentially with barding combined with full plate armour and shield is too good to pass up. Both have access to a massive amount of mundane equipment though alas in 8th edition the option for a halberd has gone (which was a favourite ofmine for heroes).
The imperial pegasus can be a good choice as if you join units of demigryph knights you are now the same unit type are no longer a target if the unit is big enough. It also allows you a lot of tactical flexibility be able to zip off and target wizards and war machines though of course the benefit of "hold the line" may outweigh this.
Pros and cons
A general of the empire is an easy (and cheap) way to get leadership 9 for your troops this can be increased to 10 by placing him in a unit with the magical banner the standard of discipline, given empire have an average leadership of 7 this is an important boost. With the benefit of the detachment rule and there cheap points cost it is easily possible to have more then 1 to spread the leadership and hold the line special rule to several unit across the army.
Although cheap this comes at a cost empire captains and generals are mediocre characters at best with nearly ever other army having superiour characters, combat wise both generals and captains have the same stats. This means against powerful enemy characters and units you are looking to your armour and magic items to stay alive. With the easily obtainable 1+ save killing blow is a serious threat to these characters but weaker attacks will find it tough to get through.
One con is if they are mounted on the imperial pegasus or imperial griffon or you are running them solo (particularly the griffon as they can never join units ever) you lose the benfit of the hold the line special rule as you are no longer accompanying a unit (obviously having someone to shout at helps increases the characters resolve). You are of course paying a few extra points for this ability as well.
The higher leadership you can get in an empire army the better though an arch lector matches the leadership of a general of the empire both have different abilities, of course with the low cost of empire characters it is easy to have both (a grand master is another option though double the cost of a general). The "hold the line" special rule when combined with stubborn/steadfast and/or the battle standard re roll makes empire units extremely hard to break. Considering they are not particularly amazing in combat (though every little helps) this is the main reason to take them.
The cheapness of empire characters also allows you to split roles across numerous characters unlike some elite armies who will combine these in one character this makes them less of a target and on occasion ignored all together leaving them to get on with their job.
One captain of the empire should of course carry the army battle standard the leadership re rolls it grants are invaluable in a game and it is generally best to make him as hard to kill as you can. the fact he also gives at least one unit hold the line makes it an excellent investment for the points cost.
A captain of the empire riding an imperial pegasus is also a good choice for hunting down small units such as fast cavalry, wizard and war machine hunting and is potentially hard to kill with a 1+ save easily possible with the helm of fire or the enchanted shield.