These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Be Careful What You Wish For: Fans were hoping that Ward wouldn't be writing Ultramarines in the next books well they're right, only problem is Robin Cruddace (writer of 6th Edition Codex) more or less kept everything Ward wrote, fan favorite Phil Kelly (who wrote the Damnos Warzone book) took the "Ultramrines are awesome" Up to Eleven.
Broken Base: On the Ultramarines being one of the more "noble" and "heroic" chapters. This hasn't been helped by Matt Ward exaggerating both qualities.
Also on Uriel Ventris and his series. Some Ultramarine players enjoy his deviations from the codex and his travels, while others dislike the near-exclusive focus on him and would prefer a more standard Ultramarine protagonist.
Fridge Brilliance: Why can Pasanius survive a point blank shot from a meltagun which should have otherwise killed him? Because the first novel noted that, due to his size, his armour was made from a heavily damaged suit of terminator armour.
Boring Invincible Hero: Many fans complain about how ridiculously skilled some of the higher ranking Ultramarines are, what with a captain that's earned more medals by himself than any other entire chapter, and a scout sergeant who's responsible for the Ultramarines "having the best marksmen in the entire galaxy".
To make matters worse, Matt Ward, the author of the most recent codex, has taken to ramming how awesome all Ultramarines are down peoples throats. This is done to the point where one small paragraph notes that Ultramarines scouts can have the same level of experience as most captains from other chapters by the time they don power armour. The second company is also described as being the "greatest fighting force in the Adeptus Astartes."
Graham McNeill actually tried to tone down some of the more overt fanboying in Chapter's Due. Matt Ward then tried to retcon some of it out.
A problem some have with Mc Neill is that they feel it takes it too far in the other direction.
Thankfully the 6th Edition rulebook removed this. Chapters that don't follow the Codex Astartes are no longer considered "aberrant", other Chapters don't look up to Guilliman as their "spiritual liege", and pretty much every other Mary Sue trait has been done away with (minus the bit about the Honor Guard).
Mary Suetopia: Ultramar is a little empire of prosperity, especially compared to the rest of the Imperium. A lot of this is due to the Ultramarines' rebuilding strategies during the great crusade. The fact that there are seven civilized worlds whose only responsibility is to support the Ultramarines* Other Space Marine Chapters generally have only a single world supplying innitiates and supplies, and it's often a savage world or a death world. And because the planets of Ultramar are responsible for maintaining the Ultramarines, the only tithes they pay to the main body of the Imperium are gene seed tithes, which have no effect on the common population. Normal worlds, on the other hand, have to tithe some combination of food, supplies, manpower or the like, which can place a lot of stress on the population during hard times helps a lot too.
It's actually brought up in The Chapter's Due, where it's noted that Ultramar is a warrior society who disdains people who don't pull their weight, and also lacks the crushing hopelessness of the forge and hive worlds so common throughout the Imperium.
To the Imperium it is a Mary Suetopia. To you or I it would appear more like Rome.
Moral Event Horizon: Honsou crosses it with destroying Tarsis Ultra. Leto Barbaden and Verena Kain crossed it with the Khartuian Massacre. Prelate Culla crossed it with beating Mykola Shonai to death with his bare hands.
My Real Daddy: Amongst those Ultramarine players who dislike McNeill's take on the chapter, Aaron Dembski-Bowden and Dan Abnett are considered this.
And even they agree he's this compare to Ward.
Narm: The various Imperial Fists corpses arranged as increasingly elaborate wall-art. It doesn't help that their yellow armor really brings much needed color to an otherwise drab planet.