Lolin NodeMCU V3 and 5V sensors
What about the amperage? (I guess that the max amperage of the USB power supply is divided between the board and sensors/actuators connected). How does it compare with the VCC output of, let say, an Arduino UNO?


The 5V pin is connected directly to the 5V of the USB. Whatever your USB port can provide the 5V can provide, minus the current used by the ESP8266.


  How important is in most practical cases not having exactly 5V? (clearly, I am not considering using the sensors for extremely accurate measurements). I am interested in things like movements/distance/light/accelerometers/humidity etc. sensors and LCD displays, relays, small motors, etc..


That all depends on the device. Read the datasheet. Look at the Electrical Characteristics. It will tell you the recommended (and the absolute maximum) voltage range.


  Is there a better yet simple way to connect 5V devices to this board?


Again, it all depends on the devices. Many many devices that are marketed as 5V are in fact "anything from 2.8V to 5.5V" or something like that. Read the datasheets!

The NodeMCU is a tricky board to use with 5V because there is no isolation between the USB power and the 5V pin's power. You can't power the board through the 5V pin and still be able to use the USB port or you risk destroying the USB port or computer that the NodeMCU is plugged into.  If possible you really want to be running as many 3.3V devices as you can, or to keep any external 5V supplies completely separate (except ground, of course) from the 5V of the NodeMCU.