Combination moisturiser-plus-sunscreen products are a useful marketing category and a poor protection strategy. The maths is the problem, not the product itself.
The two-finger rule and what it implies
SPF testing assumes 2 mg per square centimetre of skin. For an adult face plus neck, that's roughly two adult finger lengths' worth of product — about a teaspoon. Almost no one applies that much moisturiser; most use a pea-sized amount. At a quarter of the tested dose, you get roughly the square root of the SPF: SPF 30 becomes SPF 5.5.
Dedicated sunscreens are formulated to feel applicable at the proper dose. A teaspoon of moisturiser on the face feels heavy, slow to absorb, and obstructs makeup. People simply don't use that much, so the SPF claim becomes theoretical.
When SPF moisturiser does make sense
Indoor day with one short outdoor errand
A moisturiser with SPF 30 used as moisturiser (not as sunscreen) gives a small protection boost over nothing. Better than zero.
On top of dedicated SPF
Some people layer SPF moisturiser under a separate sunscreen for makeup feel. Doesn't add up linearly but won't hurt.
Sensitive skin with SPF intolerance
Some users tolerate a tinted SPF moisturiser when full-dose sunscreen breaks them out. The reduced protection is still better than skipping protection altogether.
Picking a sunscreen that you'll actually apply properly
Texture matters more than SPF number. Anessa Perfect UV Sunscreen Skincare Milk is the gold standard for hot weather. La Roche-Posay Anthelios UVMune 400 is the UK fluid that absorbs cleanly under makeup. Beauty of Joseon Relief Sun is the cheap, comfortable Korean fluid most users actually use enough of.
If the texture stops you applying a full teaspoon, switch products until the texture works. SPF 50 you apply at half-dose protects less than SPF 30 you apply at full dose.
Reapplication: the harder problem
Once outdoors, SPF needs reapplication every two hours. Powder sunscreens (Supergoop! (Re)setting 100% Mineral Powder) or sunscreen sticks (Beauty of Joseon Matte Sun Stick) make this realistic over makeup. Spray sunscreens (Garnier Ambre Solaire mist) work if used heavily — most aren't.
Use a moisturiser as a moisturiser and a sunscreen as a sunscreen. Combining the two saves a step but costs you protection in a way most marketing never explains.