When you explain an update command, it only shows how the query part is executed; it doesn't show anything about the update part.

> db.c.explain().findAndModify({_id: 123}, [{$set:{x:45}}], {upsert:true})
{
	"queryPlanner" : {
		"plannerVersion" : 1,
		"namespace" : "test.c",
		"indexFilterSet" : false,
		"winningPlan" : {
			"stage" : "UPDATE",
			"inputStage" : {
				"stage" : "IDHACK"
			}
		},
		"rejectedPlans" : [ ]
	},
	"serverInfo" : {
		"host" : "dpercy-desktop",
		"port" : 27017,
		"version" : "4.5.0-970-g847dfa3",
		"gitVersion" : "847dfa31a717d9e57028de7cd39e45dc15d3b405"
	},
	"ok" : 1
}

I think the UPDATE stage should include information like:

• how the [{$set: {x: 45}}] pipeline is represented
• whether it's a multi-update
• whether it's an upsert
• which version of the updated document will be returned (new, old, or none)

Also, if possible:

• which indexes will need updating

This last item could be helpful for deciding whether you have too many indexes. If an update is slow, you .explain() it and see all the indexes that are slowing it down.