The way it was explained in the pain seminar I attended was that overuse of
opiates leads to more pain for those with Central Pain Syndrome.
These drugs escalate the nerve loop in the brain, leading to needing more
drug..becoming a vicious cycle.

For others, the Dr. Clauw said for example, arthritis pain, inflammatory pain in the periphery, some NSAID or opiate may be used carefully.

Fentanyl itself is not a horrible choice, if you do not need to up the doses regularly. Constipation is the worst side effect to deal with daily. If you are not sedated by it, then that is not a factor.
Long term opioid use can suppress the pituitary axis, but reports on this differ.
Many studies were done on methadone ( and not fentanyl). It appears worse in men. Women past menopause don't have estrogen anyway.

How you broach this subject with your doctor, is 1/2 the battle. If you are not increasing, that would at least be a bargaining chip. As soon as you need to increase, I can see the doctor being resistant. You might be able to buy some time with her.