It's slow (execution speed [1] -- it's slower than Node.js by a lot!), and every developer that I've spoken to has given very, very long estimates (from my POV) for accomplishing even trivial API work, so it seems to be prone to low development velocity. It's possible that every single Java developer I've worked with is nigh incompetent, but they had jobs at places like Oracle, Amazon, and high-end consulting companies, so I'm not sure where the strong Spring developers are hiding.

I don't know the internal architecture, but given the poor benchmark results, I'd hazard a guess that Spring also functions as a thread-per-connection server, which means resource use would be very high for real-time applications (compared to Node.js or an event-based Java framework). And most applications today require at least a real-time component, so if you need 10x-50x as many application servers because you need an OS thread per concurrent connection, that's a huge additional cost.

[1] https://www.techempower.com/benchmarks/ -- Java and C++ tend to be the fastest in the benchmarks, but Spring is so slow that it hits between 2-16% of the top platform speeds, depending on benchmark.