I worked on the Handel-C compiler :-) Then later used the language for a few years.
Its approach was intentionally simple conceptually. You could tell at a glance how many synchronous clock cycles each step would take, and roughly what logic would be produced, so it worked quite well for deterministic I/O and simple logic.
I found it a bit of a pain for high-throughput pipelining though, and personally prefer a compiler that has more freedom to auto-balance pipelines and retime logic.
I think Handel-C occupied a middle ground between other HLS, and Verilog/VHDL. It had the concise, C-like syntax of the former, with the predictability of the latter.
What happened to it was it transitioned from university to spin-out company Celoxica, and then was eventualy bought by Agility; then Mentor Graphics bought Handel-C while Agility folded, and Mentor seemed to mothball it.
For a while in the middle there was a decent business with great customers and a decent market cap, and something I'm not privy to resulted in the business folding. I don't think it failed due to insufficient code optimisation :-)