MLPs are required to pay profits to their limited partners. Dividend stocks can choose to stop paying dividends. Corporate bonds payments are fixed interest (a set number) but MLP payments are usually a percent of profits (a variable number). Would MLPs be between corporate bonds and dividend stocks on the risk/yield spectrum? Safer than dividends, riskier than corporate bonds. Higher yield than bonds, less yield than dividends.
On the contrary, MLPs have actually performed worse then dividend stocks in recent years. Yes their payouts (as a percentage of cash flows) are guaranteed, but remember that the value of stocks/units includes not only the dividend payout but also the value of the underlying company. A dividend payout is essentially cashing out a portion of the company and giving it to the shareholders, which means that the company CANNOT use it to grow, or worse, to weather bad financial times.
Non-MLPs can suspend dividends if necessary to retain cash to stay alive, while MLPs typically cannot. If an MLP hits a rough patch, they need to raise money somehow (usually by borrowing money), which may make them worse off financially than if they were able to suspend the dividend.
As an example, as oil/gas production dried up in 2014 the revenues of many pipeline MLPs plummeted (since they were's transporting nearly as much product).