rarely had problems with contrast adjustment, but i had one who needed negative voltage on the contrast pin, instead of the regular models trimpot between 5vdc and ground ...
Yep. Extra-large ones and ones with extended temperature ranges often require a negative voltage.
Similarly, a regular one may need a negative voltage when operating at 3.3V instead of 5V. (“3.3V” models are actually just 5V models with an ICL7660 charge pump on the back to generate the negative contrast voltage. The same board is used on 5V models, just with the 7660 unpopulated, and with different jumpers installed.)
and the other surprise could be the lcd backlight led based or ac voltage ??
Have you ever seen one in the wild using an AC backlight? (What was it? Electroluminescent? CCFL?) Those are common on bigger LCDs in the past, but the character LCDs seemed to always be either no backlight or LED. Originally they’d just have the yellow-green LEDs, then in recent years white became common. (And later ones using reverse contrast, i.e. light characters on black background, used any color LED that existed at the time.)