Um, I don't think the problem here is shounen ai vs yaoi. Sorry if I'm wrong. I don't actually read this, but sometimes I happen to see the comments in the feed, and from what I've gathered, what people are complaining about is that basically no feelings at all are shown? Like, usually in shounen ai we get cute moments and fluffiness; nothing over the top, just at least something that makes our heart flutter. If after 40 chapters there's still nothing here, then I'm sure I'm not missing out on anything by not reading it.
I think I disagree with the point of 'no feelings at all are shown'. Quite the contrary. Both of them have shown jealousy. Both of them have shown they care for each other despite the bickering.... Both of them have shown they are attracted to the other. They definetely are not indifferent towards each other.
The thing is that this is subtle --- what exactly are readers expecting as showing feelings in a slow burn story? Juna probably is more aware of his feelings for XiaoXiao than the other way around but just now he is smarting up. XiaoXiao cares for Juna even though apparently he thinks he dislikes him ... however he is shown to pay attention to Juna and instinctively wanting to please him ( like when we see that he had noticed that Juna needed a new tip for his guitar and he caught himself thinking about buying Juna a gift --- something very thoughtful, that he actually knew what Juna would like).
There is PLENTY there ... but not if what you want to see is straight and blunt show of affection. It is a slow burn with the exactly right amount of progression. Make the effort to understand the subtle hints of care and attraction between them that the author is putting in each chapter, especially from Juna towards XiaoXiao. If you are waiting for a confession or a physical sexual touch, it won't happen so soon but pay attention at how closer they are getting to each other and the small signs of mutual interest.
For the people saying that there's no action or flirting in their relationship even though it has been nearly 40 chapters, this is a shounen ai.
Yaoi manga tends to have more fan service and sex in contrast to shounen ai manga tends to focus on the relationship,bonding of the couple,and characters more. If you are looking for the sex/fan service, you shouldn't be reading shounen ai in the first place and should read some yaoi manga.
No hate on yaoi manga because i personally do enjoy some spicy fan service once in a while but i prefer if people don't mix those two genres up.