Yes, the NES requires significant modification to get RGB or HDMI output; but the Jaguar just needs an RGB cable.
If you want an all-in-one solution out of the gate, get a Framemeister. It’ll be about the same price as an OSSC plus an RT2X; it takes composite and S-Video, as well as RGB (via miniDIN), YPbPr (via D-Terminal), and HDMI; and it has a bit more capability, in terms of scaling and image normalization for compatibility with TVs, but at the cost of very slight lag (Is it only 1 frame? I don’t recall) and somewhat subpar handling of 480p.
If you do spring for a Framemeister, make sure you get one with the SCART adapter (Do they call it EuroSCART?), not the JP-21 adapter; they’re not compatible, and mixing the two can damage your consoles and equipment.
If you just want a simple device that will let you play your composite and S-Video consoles on your HDMI TV, and you don’t care about RGB or YPbPr, and you can get over the fact that composite video will look bad regardless of what you do (Use anything better when possible), you can always just use the RT2X on its own.