Since 1987 and my first exposure to VHDL, I have seen the humble VARIABLE relegated to the backwaters for use only as combinatorial logic. Verilog coders seem constrained to do the same.
Aspen Logic Journal
The Aspen Logic Journal provides articles with insights into FPGA logic design, tools, technology, and techniques.
The VHDL PROCESS, written with multiple WAIT statements, provides a powerful way to develop implicit state machines (ISM). The ISM lacks an explicit state variable to track the current state thus making it easy and quick to add, delete or modify states with simple text edit commands.
The typical article on metastability in the flip flop synchronizer (used for clock domain crossing) delivers volumes about the MTBF ("Mean Time Between Failure") equation but falls short when explaining practical ways to manage the failure rate.