My long (more than 10 feet) unsuspended medium-cargo-hauler CrazyBike2 was a lot better than a regular unsuspended diamondframe bike, but it got way better when I added sort-of-decent front suspension...but the rear was still a very small wheel (20" vs 26" front) and especially when loaded could beat the crap out of anything in it (and the rear wheel, and even broke the actual frame at one point) on our not-well-maintained-streets.
Similarly, my 11 foot long SB Cruiser trike sucks on rear end hits, though even with no suspension in front it's not *too* bad, partly because it's like CB2, 26" front, 20" rear (well, effectively 22" rear with the big tires).
Suspension, even minimal, would make those rides much better, and good suspension would probably make them a dream.
So...yes, an unsuspended longbike will feel better than an unsuspended diamondframe/shortbike, and will vary based on design and materials used, but....
As far as "enough suspension", that would depend on the road/etc conditions one rides thru, and one's expectations.