Is it usually accurate when your instinct is telling you that something is off in a relationship?

Most times, yes. You just have to pay attention to signs, etc. For example: if someone continually tells you that you mean so much to them, they live you, they feel close to you, you're important to them and you're in their heart ... But, then you either see the same words written to someone else or you overhear them telling another the same ... Then those are signs and red flags that something is definitely off.