Yeah the training arc is really hard to follow and seems like more glorification of eren that's hard to actually understand (seriously "he can do this thing that you can't even think up a real life equivalent to with gear that doesnt fit him" is not a good way to show the strength of a character, especially when ppl around him are doing so much better. also lol at describing the main 10 and then getting to eren and saying "he sucks buuuuuuut he tries really hard wow look at him wow!!!!!!!!")

as for the manga, it's just like any other genre I guess where slip-ups are expected but to the degree they happen in SNK it's almost head-scratching. like... it's just the basics of story-telling. cmon.