I honestly had not heard of CUBA until just now. So I went to check it out watched a video or two...and I still can't say that I know enough about it to authoritatively compare it with Spring/Spring-Boot. But if you're willing to accept an answer based on virtually no knowledge of CUBA, I'll answer anyway.
From what I can see, CUBA offers a modular/component-oriented development model that's heavy on forms (web-based or IDE-based) to produce code. As I watched the getting started video, I couldn't help but think about the "Evil Wizards" described in Dave Thomas and Andy Hunt's "Pragmatic Programmer". But again, having no hands-on experience with CUBA, I'll stop short of saying that what they have is a bad thing. It's just not my cup of tea (or Java, if you will).
Moreover, some of the component-oriented model reminds me a bit of what web frameworks like JSF and Tapestry offer, but with CUBA, it's for the entire stack, not just the web layer. Again, nothing wrong with that. But even though I saw some code along the way, the bulk of the development time was spent in forms and wizards tying things together. I'm a coder and--this is just my opinion--that kind of programming model is painful to me. And, in my experience (including one I'm living through right now) when that kind of model is in play there's not as much room for customization. If you want to do exactly what the components offer, that's great. But it gets increasingly difficult to move beyond the baked in functionality.
But again, I have only watched a video and have read a bit about CUBA. Maybe they've solved that problem and made form-based and component-based development super-flexible. If so, then my concerns would be less.
Spring, on the other hand, doesn't define many pre-baked components...at least not at the same level. Instead, Spring leaves it to you to define the components that your application needs and it wires them together. Yes, there's more coding...but again, that's my jam, so I don't have a problem with that. And Spring Boot makes it even easier by auto-configuring the framework-level bits so that you can focus on the functionality your app needs.
In short, I don't know much about CUBA. It looks interesting, but doesn't appeal to me much on the surface. I won't say it's bad and maybe it's a perfect fit for some projects. But for me, I prefer a framework like Spring (and Boot). And yes, I'm biased. Did you expect otherwise?