According to Mine Safety Appliances, a major manufacturer of scuba tanks, the highest internal temperature that is produced by rapidly filling a tank is about 116° F. That is from 0 to 4500 PSI in 45 seconds or a rate of 100 PSI per second. This is starting at 75° F using standard breathing air. The size and composition of the tank makes no significant difference in the amount of heat produced, it is safe to assume this applies to PCP tubes as well. This fill rate is not considered harmful to the tank. Insurers of scuba and paintball establishments do not consider it unsafe. Rapid filling of a tank or gun will not cause it to explode as long as it is properly maintained, the safe operating pressure is not exceeded and an unsafe substance is not present internally.
Some older tanks are/were made of 6351 aluminum. It develops stress cracks when pressurized for long periods of time. Use of that alloy for high-pressure tanks was discontinued by 1988 in the US. There have been very few tank explosions over the past few decades. The causes of those are not well documented. It is suspected those tanks were out of certification, corroded and/or critically overfilled. Another possible cause is a flammable or combustible substance was present internally causing rapid compression detonation.
Airguns are designed to withstand 3 or more times the pressure they will contain. As far as professionally built guns are concerned, I have never heard a report of one exploding due to a design flaw or material failure. Outside of a couple suspected hoaxes, there was a highly publicized incident where one man was seriously injured. He was taking his gun apart with nearly 2000 PSI in the tube. He used one Discovery valve screw instead of the three required when he customized the gun. He didn't realize the screw had failed and the valve was only held in place by the front trigger frame screw it had caught on. The transfer port moved out of alignment when the valve screw failed, he was taking the gun apart to investigate why it was shooting poorly. When he started to remove the front trigger frame screw the gun blew apart. The valve shot through his left thigh. He publicly blamed this incident on the HiPAC unit he was using. He had modified the HiPAC so it pressurized the tube. It was originally designed to isolate the tube from being pressurized. The modifications he made to the valve and HiPAC would have required three Discovery screws to safely secure the valve.
Airguns in general are very safe. Customizing one is also safe if you know what you are doing.