Hello! I made a similar bug list for Rome 2 a few days ago and now it is Attila's turn! I have found a few solutions (with modding) to a few of them with the contribution of good people who took a look on the report, so I hope this one may help too!
Since the game has a lot more and more complex mechanics than Rome 2 I am certain there are many other bugs as I see reported in other threads, but I cannot include all of them because I haven't confirmed them personally. I also added a few "problems" that may not be bugs but they surely bother a lot of players.
I sadly cannot post any screenshots right now but I may add them later. So...A) Grand Campaign
1) Nordic admirals do not receive melee damage bonus from general's zeal (this bug has a known fix and I am working on it)
2) Carpenter building does not provide Hull Strength bonus to new naval recruits, nor existing naval units. (bug confirmed by many people. Many cities and seas were tried but none work)
3) Some household items do not provide their bonuses (example: Pankration Gloves in GC)
4) Chapter objective bugs: The Eastern Roman Empire is especially affected by chapter objective bugs. The objective of the first chapter "Maintain at least 75% Greek Christianity in the specified province: Thracia" is bugged. When the campaign starts the specified province is visible, but when the game is loaded or a battle is fought, the province's name in the objective is invisible and the objective cannot be completed. But even if you do not load a game or fight a battle and manage to keep the province's name visible, the objective will still not be completed even if you maintain the percentage of the religion in Thracia.
5) Horse armor: Tagmata and Vandal warlord had horse armor but now they don't. The horse armor is visible on the unit card.
6) Invisible daughters: many factions suffer from this problem. A single daughter in a family tree may be invisible but still be available for marriage to other nobles or factions. The big problem is when the daughter "elopes" and gets married to another noble. Then the couple should be in teh family tree but instead they are both invisible and thus the player is unable to use any intrigues to or by the characters. Unfortunately even visible single women of the family tree may become invisible after marrying them to a noble, along with the noble.
7) Insulae orientalis and insulae occidentalis provinces do not receive seasonal effects!! (the icon indicating the season is also missing)
8) Traits get lost randomly (intended?)
9) The chance of Huns forming tributary states must be increased to avoid late games with too few factions left
10) As above, the survivability of historically important horde factions (Visigoths, Ostrogoths, Suebi, etc) that cannot be re-emerged must be increased to avoid an empty late game. Two solutions I can think of are: lower the aggressiveness of these factions and increase their tendency to camp and recruit units rather than just raid and stand idle and/or allow them to re-spawn like the Huns but at a less menacing way of course. Peace treaties should also be more common, I have personally never seen the Western Roman Empire sign a peace treaty with the A.I.
12) When Attila becomes faction leader of the Huns and is engaged in Diplomacy he has no unique voice like the trailers. This was quite a disappointment, I expected unique voice and dialogue from Attila.
12) Stat issues: I understand most units stats and I am glad some of them are the way they are. But units like Palatina guards (WRE, ERE bodyguard) would really benefit from an increase in melee defence. Tagmata's nerfing was also a huge disappointment, I bought the game after the nerfing so I don't really know how superhuman these guys where but right now the are too weak for an elite Roman cavalry units. And they don't have their cool horse armors! The Last Roman
1) The melee damage bug affects multiple bodyguard units (Ostrogoths, Vandals, Visigothic admirals) but thankfully the source of the problem is known and I working on a fix.
2) The region Thagaste of the province Zeugitana (in Africa) does not receive factional colour after conquest on the Diplomacy/Overview map
3) Ostrogothic garrisons in walled cities are too small, making the player's (Belisarius) conquest of Italy too easy and underwhelming with no strategy involved
4) When playing as Belisarius your role as a liberating military expedition is lost by the huge amount of Justinian's armies being sent to the front of the war because many times they end up doing all the conquests rather than just garrisoning the cities. This is especially annoying during the conquest of Italy.
5) Public order should be a bit higher after liberating cities for Justinian to emulate the liberation of the Roman people living there.
6) Gothic warband of the Visigoths become available from the last technology of the military tech tree which is too long. The units was available way earlier in Grand Campaign, 100 years ago.
7) Characteristics of some generals like beards are sometimes missing from the icon of the same character when it is seen in the family tree or vice versa.
8) There is no reward, cutscene or praise from Justinian when capturing the city of Rome, a huge disappointment for the player. Instead, the player is heavily insulted by Justinian just because he didn't save an Ostrogothic queen, a queen whose death historically was the casus belli for declaring war against the Ostrogoths, in order to regain the lost Roman lands. Instead, saving the queen automatically signs peace between Ostrogoths and Romans, leaving the player with nothing to do and unmotivated to invade again.
9) Chapters change too fast since they depend on date, leaving the player unable to complete the chapter's objective even though he would had if he had just a few more turns to do so before the date is reached. It is obvious that the player should not complete all of them (many are not compatible with each other) which is ok, but still, too few can be completed.C) Age of Charlemagne
1) Characteristics of some generals like beards are sometimes missing from the icon of the same character when it is seen in the family tree or vice versa.
2) Chapters end too quickly leaving the player unable to complete enough chapter objectives. It is obvious that the player should not complete all of them (many are not compatible with each other) which is ok, but still, too few can be completed. In AoC this happens due to how easily and quickly Imperium can be inceased.
3) Events sometimes include dead characters. For example even if the Lombard king is dead, he will be included in the event in which the Pope demands a city from the Lombards. Possible fix is making the king immortal until the correspoding events are over.