Multi-focal lenses for presbyopia - what are some of the drawbacks for having them?

Contacts/IOLs: Are you referring to multifocal contact lenses or multifocal iols for cataract surgery?

Multifocal lenses: Multifocal lenses are available in contact lenses and lens implants at the time of cataract surgery; many like them. They work by "simultaneous vision, " where there are 2 simultaneous focal points. You increase the range of vision in exchange for a decrease in contrast sensitivity / sharpness. Rings and halos are common at night. Reading can be very light dependent (better with bright light).

Less contrast: I am guessing you mean the use of multifocal contact lenses. These consist of a series of concentric rings every other being a near or far power. Most adjust well to these but they do lower the light input for each distance which creates a diminution in the function called contrast sensitivity. This is not something too common as a problem and affects only extremes of vision.