Living Wills
A "living will" which is a "pull the plug" document that isn't customized to an individual's preferences (probably 95%+ done by lawyers are not customized anyway) through a service like LegalZoom is probably fine, although doing it yourself you don't get the same guidance about how to use it in practice and are more likely to screw up the formal execution of the document (e.g. not having the proper witnesses and notary observe the execution, or signing in the wrong place, etc.).
But this document is usually prepared by a lawyer at little or no extra cost when you have your last will and testament done, so the cost of not screwing up the execution of it isn't great. And, lawyer drafted documents are less likely to be contested in practice, even when a non-lawyer on paper does everything right.
Someone who does their own living will also often doesn't realize the important of also having medical powers of attorney and durable powers of attorney for property which are also necessary.
Simple Wills
A "simple will" is quite another matter.
First, I've never met a layperson who doesn't think that they need no more than a "simple will" when in fact they often do, either because they are affluent, or have a blended family, or need testamentary trusts to manage property for children or young adults or black sheep or for tax purposes or because some family members are non-citizens. In general, a lot of the value of having a lawyer do the work comes from the lawyer's ability to spot issues that are exceptional and take you out of the "simple will" solution by itself. Often an issue spotted can result in larger monetary savings or a much smoother probate process. For example, a lawyer can identify cases where a probate proceeding in more than one state is likely to be required and suggest steps to avoid that expensive result.
Second, many non-lawyers have a very hard time thinking about all possibilities. They do fine thinking about what rules make sense if everybody alive today is still alive when you die and you own what you own now when you die, but have a very hard time thinking about what would be appropriate if people predecease them or if their assets change substantially. Lawyers are much better at working through what is sensible in all of these possibilities, many of which won't happen, but some of which will happen. This matters because a will never expires unless it is expressly revoked. I've probated wills drafted during WWII in basic training (as required) before the decedent went off to war and never amended over the next 60 years, and it is very hard to be that thoughtful when you are doing it yourself.
Third, it is very common for non-lawyers to use language that isn't obviously ambiguous or otherwise problematic until you are forced to apply it in practice. Estate planning lawyers are much more aware of these traps in the "moving parts" of an estate plan and of the possibilities that need to be provided for.
To give one example, suppose that you leave your second wife your house, and leave the remainder of your estate to your children (her stepchildren). It is very easy to say this in a way that does not make clear whether she takes the house subject to the mortgage, or if the mortgage is a debt to be paid before the remainder of the estate is distributed to the children.
Similar issues often come up in relation to tax elections and allocation of tax debts among heirs.
Providing for the disposition of pets is another thing that few non-lawyers manage to do well.
Lawyers, in contrast, generally draft in a manner that avoids these ambiguities and sets forth rules that are sensible, fair and will work in practice. The issues are even more fraught if businesses or investment real estate is involved.
And, non-lawyers (even sophisticated, affluent business people) routinely fail to grasp that a Will only governs assets which don't have beneficiary designations and is subject to forced marital share and minimum family inheritance laws that act by operation of law as well as other "gap filling" presumptions that modify the literal meaning of certain kinds of language in a Will.
Finally, screwing up the execution of a Will is very common, while lawyer drafted wills are much less likely to be contested.
In my twenty years of experience as a lawyer who does estate planning as part of his practice and teaches lawyers, financial planners and paralegals about the topic, I find that the increased litigation costs associated with a do it yourself will (on average) is about ten times as large as the savings associated with doing it yourself. Sure, one time in three or four or five, somebody does their own will and doesn't screw it up and it all goes fine, but a majority of the time, do it yourself will drafters do something that would be considered malpractice if a lawyer did it. Pay lawyers now, or pay lawyers more later.
Honestly, if all you need is a "simple will" and you are not willing to spend the $500-$2,000 to have a lawyer draft appropriate documents, and help you execute them, you are probably better off doing nothing at all and dying intestate (i.e. without a will so that the default provisions of the law apply), which often isn't a horrible result in a plain vanilla, unblended nuclear family that isn't particularly affluent.
