AFAIK, the Avid Mojo SDI is an SD device similar to the Liquid Bob. It is basicaly an upgradfe from the regular Mojo DNA box which I have. The DNA box is quite nice and is capable of full 4-2-2 component output. Unlike the Liquid BOB, it cannot downconvert and HD timeline to SD on the timeline so you have to switch your HD project format to SD in order to see it out the Mojo.
For HD output, you need to look at the various Nitris boxes. They are expensive. If you just need HD monitoring to a client monitor, you can use the second head of your video card and a DVI to HDMI converter to acccomplish that. If you need analog or SDI out to an HD deck, then you need to look at one of the Nitris boxes.
As far as scaling video and graphics goes, MC is quite capable in most cases but not as flexible to use. For example, to use pan and scan on a hi-rez still, you cannot just do it from the timeline, you must work with the pan and scan tool. That said, MC has some very powerful tools that do not exist in Liquid but I personally find the Liquid workflow much more comfortable although overall the capabilities of MC are much greater but there are some things Liquid just does far easier.
Many users have successfully made the transition to MC and after getting their head around the MC workflow, seem to embrace it. Download the trial version and play with it and see what you think.