Gen 4 Jimny "lethal" at speeds over 60mph....

The comments are to be expected if they are new to solid axle vehicles, most that move from modern independent suspension cars will call any "all over the road". They usually follow it up with "slow".
By comparison it's true, they do "fidget" on rough roads and speed just adds to this feeling. 
We can argue all we want that this is not the case, but those that like the Jimny (and similar) just get use to it. They aren't "lethal", you just need to be involved with the driving process, something modern "cars" are very good at isolating from the driver. 
I suspect a Jimny simply is not the vehicle for him. 
It's possible it's faulty, but it's a Suzuki so unlikely. 
I personally don't think the Jimny is bad, but that's compared to my 1960 Land Rover which is lethal and all over the road

 

"Lethal" is a rather emotional description. Perhaps there are a few factors to consider such as the shape of the Jimny, the vehicle weight, the short wheelbase, the live axles and the OEM tyres (Bridgestone and Dunlop) - all constituents of a cocktail that doesn't encourage high speed driving on open roads. Personally I'd recommend driving within the capabilities of both driver and vehicle. High winds and standing water on third world motorways is an excellent test for your mental reactions and lower torso muscles!

It's all a matter of perspective, or perhaps "point of reference" - what is it you're accustomed to driving.

If all you've ever driven is a front wheel drive or front wheel drive based, all wheel drive, sedan/hatch/cross-over/SUV, then you're quite likely to perceive the Jimny as unstable, and, yes, even lethal, especially on roads that are "not smooth" or "have lines worn in them".

If you've been around live axle rear wheel drive vehicles, you're going to see the Jimny in a very different light.

The Jimny is what it is, a short wheel base, live axle, body-on-frame, rear wheel drive (when in 2H) vehicle and that is exactly what it drives like, and anyone who has been around vehicles that fall into this category should know what to expect before they even get behind the wheel.

Our JB74 was purchased by my wife & daughter for my daughter's use, and before the purchase I gave them a heads up and that included the statement that it would not be suitable for long distance "cruising", which to some extent can be seen as "paralleling" the "avoid motorways" comment, except that we don't have any roads that can be considered "motorways" - their response to that was "we have other vehicles for that".

With nothing more that a tire change (from the OEM GrandTrek AT20 to General Grabber A T/X in a 215/75R15), they have been quite happy with it, and that does include some relatively long trips, two & a half hour drives, which have required them to slow down at certain points, because of poor road surfaces that were not a problem for the second vehicle in the convoy (a 2005 Grand Vitara).

There has been the odd comment from my daughter, things like the Jimny really shows that certain roadways are really not that smooth, with the point of reference being her having repeatedly driven that stretch of road in my 98 Grand Vitara - the only negative comment I've heard about the Jimny is that it is noisy, again compared to the 98 Grand Vitara.

The ladies who primarily drive that Jimny are not critical of the vehicle for the way it drives, they perhaps recognize it is our "third world roads" that are the problem.

I get this comment alot from people when I say I own a Suzuki and they are like oh its the one that falls over which is simply not the case and I explain how that came from the consumer reports in America trying to kick the samurai out because they don't like vehicles that are better than their own *cough* Jeeps.

Fordem is spot on regarding point of reference. I can definitely see why some people would find driving a Jimny unnerving. By modern car standards they are undeniably terrible to drive. I have no issues driving my 20 year old Jimny at 15mph above legal motorway speeds, but then I've grown up driving farm machinery, sliding cars around fields, doing track days and riding motorbikes so my point of reference is very different to someone who's only ever driven normal cars on normal roads at normal speeds.
Jimnys have a natural tendency to wander, and the more you try to fight it the worse it feels. Driving with a white knuckle death grip on the steering wheel is a recipe for a stressful drive.