The case against using life insurance as a default investment is about cost and drag. A permanent policy bundles insurance and savings, and the savings side has to carry the insurance side's expenses. Every premium is reduced by the cost of insurance and by policy charges before anything is credited to cash value, and a substantial part of the first year's premium typically goes to the sales commission, which is why cash value in the early years is often far less than the premiums paid in. It commonly takes many years for the cash value to equal the total premiums, let alone to earn a competitive return on them. An ordinary investment account has no cost of insurance and no first-year sales load of that size, so the "invest the difference" alternative starts well ahead and the policy spends years trying to close the gap.
The case for it is narrower and real. The growth inside the policy is tax-deferred, and the death benefit is generally received income-tax-free by beneficiaries, which is valuable to specific people: someone who has already maxed out tax-advantaged retirement accounts and wants another tax-sheltered bucket; a person with a permanent need for a death benefit, such as funding estate taxes or providing for a dependent who will never be independent; a business with a buy-sell or key-person need. For those situations the policy is solving a problem an ordinary brokerage account cannot. What the tax treatment cannot do is rescue a high-cost policy sold to someone whose actual need was temporary and whose retirement accounts were not yet full.
Two tax mechanics are worth stating precisely because they shape the outcome. First, the tax advantage of accessing cash value depends on the policy staying within federal limits: a contract funded too quickly relative to its death benefit becomes a modified endowment contract, after which withdrawals and loans are taxed less favorably. Second, the cost basis in a life insurance policy is generally the premiums paid, and if the policy is surrendered for more than that basis, the gain is ordinary income; a policy surrendered for less than the premiums paid, which happens when costs have eaten the value, generally produces no deductible loss. So the tax-free promise attaches cleanly to the death benefit, and is more conditional on the living side.
Because these products are sold on commission, the incentive to present a permanent policy as an investment is built into how they reach buyers. That is a fact about distribution rather than a verdict on the product: the same tax features that make a policy the wrong default for an ordinary saver make it the right tool for a genuine permanent need. The way to tell the difference is to separate the two questions the bundle combines, how much protection is needed and for how long, and where long-term savings should go, and answer each on its own terms before deciding whether one product should do both jobs.