This is a bit of a broad question. Better at what? On what surfaces?
Paint/primer combos are just high build paints with good hiding abilities.
If you're painting over a new drywall one coat of paint and primer doesn't seem to give good results. You can use the first coat of paint/primer as the primer coat but why use a $30 per gallon paint/primer as the primer coat when a $10/gal PVA primer (or more savings in 5 gallon buckets) has been doing that job reliably for years.
Painting over bare wood? Most good latex primers won't seal the wood to prevent tannins from knots bleeding through so I wouldn't count on a latex based paint/primer to do much better. There are specific primers that work well in these situations.
If you're concerned about inhibiting mold you should first look to correct that conditions that are favorable to mold growth. Up until recently paints could only inhibit the growth of mold on their surface but now there's a mold killing primer made by Zinsser who I believe was the first company that was addressing mold/mildew concerns in their paints and additive products. Personally I would use traditional means of killing mold first and possibly use that primer for some extra piece of mind.
Going over oil based paint with latex or a high gloss sheen with flat? These are applications where you should get better performance with a primer that works well for those applications.
Paint/primer products work well for going over already painted surfaces where you just want to change the color of an interior surface. The high build will make covering the old color more effective and you could possibly get away with just one coat. I personally always like to do two coats though.
Primer is not a single product. There are many different types of primers suited for different situations. If you want the best results choose the right primer for your specific application. In some cases the primer will be cheaper than an extra coat of paint/primer, in others the primer may be the same cost or more expensive but will give you better results for that specific application.