When I took estates law (from a practicing estates lawyer, so understand that she was theoretically a competitor to these products) she said that the forms are usually valid if used properly. However, because there’s no lawyer there to vet it, someone might not understand that their situation is different enough than the norm that a basic form won’t work for them.
But the real problem, she said, which had led to many form wills being thrown out, was a failure in execution. At common law, for a will to be valid, three non-beneficiaries had to witness the testator sign, and then they all had to witness each other sign as well. That’s become looser over time (the standard is now two witnesses, and if a witness is interested in the will, esp. in a minor way, courts will often just void that bequest instead of the whole doc). But they’re still quite strict, much, much stricter than the formalities required to make a contract.
So for instance if you sign your will in the presence of the appropriate number of witnesses, but then one of them goes to get a coke in the other room while another signs the witness statement, that can vitiate the will.
Wills aren’t that expensive if they’re simple, and if they’re not simple, you definitely shouldn’t be using a form. Of course I’m a lawyer too (not an estates lawyer), so you should understand my biases as well.