If I'm reading the specs correctly the Titan IIIE was a significantly more powerful vehicle than the Ariane 5 while launching a much smaller payload. JUICE also has to be going slow enough to enter orbit without carrying an insane amount of fuel for braking while Voyager just did a flyby.
- "Titan IIIE was a significantly more powerful vehicle than the Ariane 5 while launching a much smaller payload"
Yes, with a fourth stage. (I don't think it's particularly that modern rockets can't integrate such a solid-rocket boost stage, but rather there's no payload that needs this).
- "JUICE also has to be going slow enough to enter orbit without carrying an insane amount of fuel for braking while Voyager just did a flyby"
The flyby speeds are similar; but JUICE takes a time to get to that speed.
It's not really that difficult to capture into Jupiter. Parent's link [3] says it's only 900 m/s for JUICE. Part of the reason is the extremely deep gravity well; and another is that the moons are large enough to give you effective gravity assists, within the orbit of Jupiter.
