This question pertains to USB-C not MagSafe. MagSafe negotiates available power with the Dallas 1-wire protocol. USB Power Delivery (PD) is an unrelated protocol. Answers based on experience with MagSafe are unlikely to be relevant.
I'm considering buying a 2016 MacBook Pro Retina about which this article says:
the Griffin product was only powerful enough to offer 60W of power, not quite enough for the 87W needed to power the larger Pro models at fully
I'm also considering buying a LG 27UD88-W which it advertised as providing:
Mobile / Laptop Charging Up to 60W
What are the consequences of using a 60W supply instead of an 87W one?
Does it just mean it will take longer to charge the laptop?
Could it have side effects such as putting parts of the system into a slower, low-energy mode?
This question covers MagSafe power supplies but Charles Duffy's comment suggests that the rules for USB-C may be different.
Best Answer
Apple partnered with LG to create a 4K monitor that outputs 60W via USB-C. Apple has mentioned full compatibility with a 13" MacBook Pro (comes with 61W power adapter) but mentioned that with a 15" MacBook Pro, the battery will get drawn during intensive power usage and therefore it should be connected to its 87W power adapter. No mention of any dangers in doing this.