For MLP's cash distributions to owners often exceed partnership income, and when they do, the difference is counted as a return of capital to the limited partner and taxed at the capital gains rate when the unitholder sells.
Not quite that simple. Items of income and expense are passed through to partners; Those items increase and decrease your basis respectively. Distributions reduce your basis and are tax deferred as long as basis is greater than $0. When you sell, your sale proceeds minus your adjusted basis defines your total gain. Depending on the MLP, a portion of that gain will classified as ordinary income resulting from recapture of depreciation and depletion, etc. and the remaining balance is cap gain. Its fairly difficult to predict the split of ordinary vs cap gain.
Of course in a Royalty trust, depletion also reduces your basis (but not below zero) and upon sale depletion is also recaptured at ordinary income rates up to the amount of gain.
Royalty Trust is better for non-IRA since you can file Schedule E to deduct all the Royalty depletions, usually result a loss before you get all you invest cost. After that, you have to use percentage depletion for more than 20% of Royalty income. The MLP only deplete Royalty portion of the dividend ...