As someone who is at around a high B2/low C1 level in Dutch now and moved to Belgium and used dualingo in the beginning I have a bit of insight into it.
It doesn’t do shit for grammar and sentence structure, but it builds vocabulary. If you learn only with dualingo, you will probably make a lot of mistakes, flipped adverbs, verbs in the wrong place, sentence structure errors, etc…
It definitely made me feel like I could speak Dutch because I could read it MUCH better after 1/2 of the dualingo course, but then when I moved, speaking was pretty bad and I had only moved up to an A2 level with the vocab of maybe B1. You definitely cannot become semi-fluent or fluent with dualingo. It doesn’t teach, it only helps practice what you have already been taught.