I think the IVAS is super interesting. And it packs a helluva lot of sensor tech in a very small package.

However, it does not do well in low light. As is obvious from the pictures, it limits natural light, forcing the soldier to rely on the sensor systems earlier in the day than normal.
Even going into a halfdark building during the day can be an issue with shades.

Yes, you can use the light enhancing sensors, but it's still sucks to wear shades at night.

On top of that, the sensor system does not feature a normal gen3 etc. tube, so it relies purely on the CCD sensor. Old school NVG tubes (as used in PVS14/31) are still vastly superior to CCD's in the same package size.

In terms of being able to see in the dark, on scale from 0-1000, it's like this:
0-100: gen1 NVG,
100-300: Lowlight optimized CCD (small package, eg. Syonix)
200: Bad Gen2 NVG
500: Good Gen2 NVG
1000: Good Gen3 NVG
(This is a simplification. Generation does not correlate to SNR/Res so precisely in reality).

And this is why soldiers would complain. Even the pure night performance capability is serious step back. Especially when you consider the new 31 NVG's that have thermal overlay integrated (honestly this is a really good and usable product design).