Yes, Honor Holly 3 is a true 4G VoLTE mobile phone.
To add watermark, go to Camera App > Go to Camera Menu > Go to Watermark and click your picture.
If you want to add watermark on the already captured pictures, Go to Gallery > Select the picture where you want to add watermark > Click on Edit > Select Watermark option.
I have used this phone for almost about 3 months and i did not find any major drawback. The only suggetion to the company is to provide at least pair of headphones in the box.