Make sure you have installed all files properly. Detailed guide is here:
http://www.moddb.com/engines/xash3d-engine/features/running-half-life-under-xash3d-users-manual-englishGamma just have other settings than in GoldSource, so you should check Video options and set appropriate level with a corresponding slider. Or put in console "gamma 1.3", it's an average comfortable value for Xash3D.
Can't say what's wrong with Nighvision. It's definetly works for me, just maybe not so bright, like in GoldSourse, it's possibly can depend on ligthing settings in Xash3D, or it's just a gamma settings' effect. There are also some specific Xash3D's cvars which allow to adjust things like texture lighting, model lighting etc.
I've made entpatches for all maps and corrected model paths to load all models properly. Besides that I've found a couple of other issues.
1.
Error: No spawn function for sprites/lgtning.spr
NULL Ent in UTIL_PrecacheOther
Looks like precache error in dlls that can prevent to load lgtning.spr properly.
2. Those flying things can't damage me. It can be an issue of custom model (sentinel.mdl), maybe missing attachment or something else that the engine should discern as a contact point of making an attack on player. Or maybe it's a coding issue. Nevertheless GoldSource can manage with attacks without that kind of trigger, but Xash3D can't. I can try to check a model, but if it's a coding problem, I can't say anything.