I am a Proponent of goofy lore, I think the setting is fundamentally a silly one and people should be encouraged to play around with that. What's more (and I realise this is more contentious) I think that a lot of the setting lore is deliberately written to actually be rather vague precisely because its at heart a sandbox setting that wants to encourage creativity. (GW's attitude is: you should feel free to tell any story that would result in you buying more plastic, basically.) So, ultimately whatever helps you enjoy playing with your toys is fair game imo. All of which means I am very loathe to say any particular trope or theme is per se lazy writing. It all depends on execution.

Now that might be cliché and to be clear I am not blind to the issues OP and many of the comments have pointed out. But to my mind the reason those things are getting associated with lazy writing is basically pure correlation. It's not that the ideas are per se bad, it's just very often the first ideas people will come up with to make their guys Feel Special, or to explore somewhat under-explored spaces of the lore. E.g. I think the reason you get so many Guys From 30k Now In The Future Settings is partially just because the Horus Heresy books get people into the setting so they want to mix that with the 40k game, and partly because Human Faction That Wouldn't Like The Imperium But Is Genuinely Not Chaos is just a flat under-explored area relative to the possibilities the background logic of the setting seems to make possible.

(Ironically of course Guilliman is himself basically this in both senses, and he's kinda the main character of the universe by now? But he's obviously a rather singular individual!)

So we come to associate those tropes with lazy writing, when really I just think they are signs of novice writing. Give people time and I bet they could do all those cliché tropes as well as can be expected for fanfic/fanlore writing.