This Tech site explains what AMD says about the 110C Hotspot Maximum Operating Tempertures: https://www.techpowerup.com/258228/110-c-hotspot-temps-expected-and-within-spec-amd-on-rx-5700-serie...
"On Hotspot, AMD writes: "Paired with this array of sensors is the ability to identify the 'hotspot' across the GPU die. Instead of setting a conservative, 'worst case' throttling temperature for the entire die, the Radeon RX 5700 series GPUs will continue to opportunistically and aggressively ramp clocks until any one of the many available sensors hits the 'hotspot' or 'Junction' temperature of 110 degrees Celsius. Operating at up to 110C Junction Temperature during typical gaming usage is expected and within spec. This enables the Radeon RX 5700 series GPUs to offer much higher performance and clocks out of the box, while maintaining acoustic and reliability targets."
Like I mentioned you can use the 110C Maximum Operating Temperature as a reference when you overclock the GPU. Like I mentioned the GPU card has several Thermal Sensors throughout the card. If any of the Thermal Sensors detects a temperature at or above 110C the GPU card will automatically start to throttle or slow down to lower the power usages thus lowering the temperature.
Now as to being "safe" it is difficult to answer because you can cause GPU failure or damage if your incorrectly Overclock or change voltages that is not related to temperatures.
I suggest you post at Overclockers.com. There other Users can help you correctly Overclock or change voltages: https://www.overclockers.com/
