I have 15W USB-C chargers for Nexus 6P, and they report in System Information Power view followingly:
AC Charger Information:
Connected: Yes
Wattage (W): 15
Charging: No
(It is not charging in this case because the battery is full.)
My 15" MBP draws about 7-8 watts with minimum display brightness, applications running but idle, on integrated graphics. At full display brightness, power draw is about 14 watts. With this laptop, 15 W charger is barely sufficient to maintain light work, but I haven't dared to try out how long charging a battery would take, even with laptop sleeping.
My assumption regarding "underpowered" USB-C chargers is that laptop draws as much power from them as necessary, and if the load is higher than what USB-C charger advertises to supply, battery is discharged for extra. (I base this assumption mostly on amperage readings seen on System Information Power view.)
Behaviour with almost empty or completely empty battery is a mystery, though; my guess is that CPU may be heavily throttled, but at some point, system may simply not be able to keep in limited charger power budget (for instance, with discrete graphics powered on), and simply goes to suspend instead, until a sufficient power supply is connected...