I make this observation not to criticize the UN; it is, as Rex says, a wholly owned subsidiary of the member states. Thus a UN PKO can only be what its members want it to be. As Jon says, UNOSOM II was a case in point although I disagree as to the reasons it failed. Created under UNSCR 814, it had the most robust C2 structure of any Peace Op to date (and maybe ever). At US insistence the SRSG was an American, former Dep NSA VADM Jonathon Howe, the FC was a NATO general, LTG Cevik Bir from Turkey, and the Dep FC was again US MG tom Montgomery, dual hatted as COMUSFORSOM. From may until June 93 the major problem was that DPKO was unable to either negotiate sufficient force contributors or get them deployed in time so the UNOSOM II force which was to have been 28,000 strong at handover on 4 May was only 14,000 at it peak. Then came Aideed's attack that kileed several peacekeepers.
At Admiral Howe's insistence, with the concurrence of USUN Rep, Madelaine Albright, the UNSC passed over the weekend, UNSCR 837 which authorized operations to capture Aideed and any others who assisted him putting UNOSOM II on the side of his opponents. This brought into play the problem of Terms of Reference (TOR) - the agreements signed by DPKO with the force Contributors specifiying the rules of engagement and precisely what they agreed to do under the terms of UN "Operational Control" (which, at best, is the equivalent of US tactical control - TACON). Italy, which both contributed forces and provided the U2 - intel staff officer - had long standing relationships with Aideed and said flat out that 837 was not what it had signed up for; Italy had only agreed to enforce 814. Hence conflict.
I won't go into the US C2 fiasco other than to say that it violated our own doctrine in every possible way.
If interested see Chapter 12 of Max Manwaring and my book, UNCOMFORTABLE WARS REVISITED and/ or Chapter 9 of my edited volume, THE SAVAGE WARS OF PEACE (thesource of the other chapter written with Tom Daze who was XO to Montgomery).

Cheers

JohnT