I bought a Streambar to replace an Ultra and replace two sets of cords with one set. The description of the Streambar stated ~Ultra Inside so I thought good. Well, navigation is much slower and remote included doesn't Fast Forward YouTube but my Pro remote does. Why doesn't Roku put 4GB in the Streambar for additional $10. Wow!
As a general rule in the audio/video world, the higher-end separate components tend to be aimed at enthusiasts who are willing to pay quite a bit for each component. Integrated devices are normally aimed at more casual users and are more focused on ease of use and the devices are typically much more constrained in the budget for each internal component.
Here's a handy page on Roku products.