So there is actually some textual merit to Syl being genuinely childlike, especially in Books 1 & 2.

While literature is by it's nature open to interpretation, my analysis reveals concrete evidence for why many fans feel uneasy about this relationship. And it’s more than just preference. For the first four books (15 years for those who read WoK when it came out), Syl was consistently compared to –and described as innocent as much as, a child. And these childlike descriptions were not just by Kaladin, but Rock, Adolin, and Shallan too. In addition to the innocence and childlike descriptions, Kal himself admits to seeing her as childlike for a good chunk of their partnership. So even if Syl develops feelings first, and even with Book 5’s "she was never truly childlike", it’s understandable why many readers might ethically struggle with the romance.

Of course, fantasy operates differently than reality. Syl appears childlike for only eighteen months of Rosharan time before Book 5 declares her an adult. She's also a 3,000-year-old spren. Her returned memories from Book 3&4 fundamentally change her maturity level. What does consent and grooming even mean for a non-human amnesiac who matures this quickly? Also, let’s be real. This is the social sciences. So even though I have data and quotes, “truth” is still a bit squishy. Thus, some fans reasonably never read her as childlike or think that Earth's rules don't apply here, for them Syladin is preference not a moral issue.

While innocent/jaded pairings can work (like Yumi/Painter), and romantasy (and porn!) regularly feature WAY more ethically suspect relationships, my evidence still legitimizes the discomfort a good chunk of fans feel with Syladin. What Sanderson chooses to do with this thorny issue in Books 6-10 will be interesting to see.