What's the most common issue you encounter when using Antminer?

heat and noise on the s21 and s19 models
they pull 3300 watts and are loud.
those two things will be certain enough.
next is break down of gear.
break down of gear is at least 10% per year.
if you have a lot of them there is almost always one or two that suck.
The s21 XP model is better though and more effective and efficient but as you stated earlier on it's power consumption, the XP consumes 3645 W of power which is massive as well. It has 270 Terahashes per second. Since it's flaw is the high rate of electricity that it consumes, it's suitable in countries where energy cost are lower and I believe if people from these kind of countries has this improved models, they will mine more than when using the older model of just s21.
Everyone has different preferences. Due to differences in regional climate and other factors, you can select the model that best suits your needs.
What's the most common issue you encounter when using Antminer?

One of the most common failures of ASICs is overheating of chips and their failure. Usually, the further the chip is from the front fan for blowing, the higher the temperature. And ASIC power supplies do not like voltage drops in the power grid.
Temperatures can be a real headache. They vary widely from place to place, sometimes hot, sometimes cold. But is too cold always better than too hot? High temperatures can be solved with a heat sink, but what about low temperatures?
Do you cool the equipment with nitrogen?

On the working equipment you will not have negative temperatures with air cooling. If the chip is dead, it will no longer heat up.
No, we don't use nitrogen to cool the system. We use water cooling plates. Not sure which works better.
Of course, water cooling will be better than standard air cooling, but there are some problems in increasing the cost and the need for engineers. Usually, water cooling is combined with something, such as heating, pool heating, etc., otherwise it will take longer to pay off.
In fact, the local environment and climate are not bad, and the electricity bill is not high. Air cooling or water cooling can be used. It also depends on local conditions. You can choose the one that suits you according to the situation. It can only be said that directly choosing water cooling is not so troublesome.