87,
Not less effective but not as strong of a magnetic field.
They each have their place. A "bank" of magnets like you see in most stock mag control reels (Akios has the best stock system out there IMHO) provide a strong braking system and give you lots of adjustable control in various fishing situations. Some may consider these types to over-brake the spool but man it is nice when you are casting into a 15-20 mph headwind, just dial it up to MAX and cast as hard as you want.....
The mono mag single threaded post mag control gives less braking overall and a wider range of "roll-off". This is perfect for field casting and tournament work (just a sinker, no bait) which is usually done with a tailwind or no wind. You dial it in to give just a slight "fluff" on the initial hit and then roll off as the sinker reaches the apex and starts down. By the time the sinker lands you can usually have the mags rolled all the way off. It lets you ride the hairy edge of disaster and tweak every yard out of a cast. The mono mag can be used on a fishing reel but usually (unless the single mag is big) will require other types of braking (brake-blocks, heavy oil, educated thumb) to maintain control, especially casting into a headwind.
Hope this helps,
Tommy