A holographic will is handwritten and signed by the person making it, typically without witnesses. Recognition is not universal — some states admit them, some accept them only in narrow circumstances such as military service, and some reject them outright. Whether one is valid depends entirely on the law of the relevant state.
Even where permitted, holographic wills tend to create more litigation per document than any other form. Handwriting has to be authenticated, sometimes by an expert, and without witnesses there is no one to confirm the testator understood what they were signing or acted free of pressure. Anyone unhappy with the outcome has an obvious line of attack.
The content is usually the bigger problem. People writing by hand tend to be informal and incomplete: they name a few specific gifts and say nothing about the residue, omit an executor, fail to address what happens if a beneficiary dies first. A court then has to fill the gaps using default rules that may bear no resemblance to what was intended.
Holographic wills are best understood as emergency instruments — something written in a genuine crisis with no access to alternatives. If one exists, it should be replaced with a properly executed will as soon as circumstances allow, not left in place because it technically qualifies.