One note: we are a small bootstrapped team of under ten, so we did not deploy a heavy HRIS. But I have run the HR function, and the way I measure whether an internal people system worked is the same regardless of scale, which is whether it reduced how often someone had to ask a person to get unstuck.
The instinct is to measure these projects by the technical milestones: it is live, data migrated, integrations connected. Those tell you the project finished, not that it succeeded. A system can be perfectly implemented and change nothing about how the team actually works.
The single metric that gave us the most insight was how often a new or existing person had to interrupt someone else to find an answer. When we put our processes and the reasoning behind them into one searchable internal knowledge base, the signal we watched was self-service: could a new hire read how we handle a situation and act, instead of pulling a senior person away to explain it. When that number of interruptions dropped, the system was working. When people kept asking despite the tool, the tool was not really adopted.
The mechanism is that the real value of any people system is removing dependence on a single human's availability. A flashy dashboard that nobody self-serves from is a cost, not a win. The interruption count is a proxy for whether knowledge actually moved out of a few heads and into something the team can stand on.
The honest part is that we observed this qualitatively rather than through a formal index, so I will not attach a percentage. What I can say plainly is that new people reached the point of contributing without hand-holding sooner, and the senior team stopped repeating the same explanations.
My advice is to measure these projects by behavior change, not technical completion. Track how often people still have to ask a human for something the system was supposed to answer. That single signal tells you whether the integration actually landed.