Comment by duskwuff

> I suspect a lot of the complication here is ultimately due to what the hardware allows

Most likely, yes. Reading between the lines a bit, there are ten hardware counters - two of them wired to fixed triggers, and the other eight each having a multiplexer selecting from a different set of triggers for each counter. This is a fairly typical hardware design - limiting the number of counters and the number of triggers per counter makes for a simpler hardware implementation. (The increased software complexity is an acceptable tradeoff for a feature which is only used by some developers.)