So essentially your heir can make an attempt on your life right now, but probably wont because of the chance for success. Adding members to the plot will raise the strength and thus success chance. You can ask the ploter(s) to knock it off in their diplomacy options, this is usually successful in my experience, especially given the heir I assume is underage as you're the reagent.
Management, keeping 4 people happy is much easier(and cheaper) than 14 obviously.
Relations, holding to many duchies(more than 2 in my experience) will lower relations with your counts, also said counts will want the ducal titles. Your essentially shooting yourself in the foot by holding the duchies in this regard.
Voting, tied into 2 and 3, you need vassal approval to raise crown authority or other such laws. Don't expect to get anything passed if you're hoarding duchies.
Du Jure claims. As a king you don't get those claims(unless the ducal title is held by you). This is a easy way to expand your realm w/o having to fabricate claims. It will also improve your relations when you help your dukes out like that(as well as give you more vassal troops)
Disadvantages:Makes those characters more powerful/prestigious.
Can cascade that power into one person, I've had vassals consolidate half my kingdom under themselves from my own poor management via marriage or war.
Larger chucks can be inherited. Depending on crown authority you can lose those duchies via inheritence if you're not paying attention. Losing a duchy is obviously more damaging then losing a county.
Dukes will want the rest of their dukedoms. Be careful when you're handing out those dukedoms if you want to keep the counties it consists of. Best example is Ireland. Most of the Irish de jure dukes are 2-3 counties, but Ulster is 4, I built my powerbase in Ulsters 4 counties and unwisly handed the ducal title to my hier, he wanted the remaining 3 counties of Ulster and sabotaged our own relations and began plotting for the throne early. If I had given him the Duchy if Meath instead he would have taken a -25 relation hit for my holding Dublin instead of the -75 for Holding Ulster, Tyrone, and Oriel of the Ulster duchy.
Theres probably other things I'm missing, but those are some of the highlights.