I do not know ESP-NOW, but I use UDP since many years (also for broadcast to synchronize a lot of network members).
There is only one trap (what I detected with ESP32, may be others with ESP8266): If the time between the sent messages is near milliseconds (or less), the messages may be concatenated because there is some delay before sending.
It is also very easy to handle because the ESP8266 WiFi library provides a typical socket interface, as you may know from networking with Windows and Linux.
Good luck ...