You can’t load into trekking poles either. The concept of loading into a long bipod is not a real thing because of physics. If you had one capable of that it would be too heavy to pack around. Also, I don’t see the javelin/long leg combo as particularly helpful without near rest. Personally, I try to get front and rear support whenever I can. Am I arguing you can reliably take a 1000 yard shot off this set up? Hell no.
Is this javelin/long leg set up a compromise? Yes. In my opinion, with a rear rest, it does is to get you a relatively stable position in cases where prone is not an option, e.g. sagebrush, tall grass, or steep slopes. Is it as stable as prone over a backpack with a rear rest? No. For me it’s a bit quicker and steadier than dealing with trekking poles and a pack because you are dealing with a couple less variables under time stress. You don’t have to loop the wrist straps and you have a consistent connection to the gun. Is the Javelin/LL more stable than a tripod? Not even close, but it’s way faster. I’ve used tripods in practice, but have had few cases in my hunting where the set up time was reasonable to get off a quick shot.
The thing Spartan touts with the Springbok is the ability to track moving game, right left or up down. I think this largely comes from African and European driven hunts, but can have some application to eastern woodland hunting. However, if you insert the javelin head into the tripod head into the socket backwards, you can get probably 30 degrees of right left tracking. I plan on using this next fall when I go back east to hunt whitetail does. As for the argument for up-down flexibility with the Springbok, I don’t find the juice worth the squeeze due to the springy head putting pressure back into the gun. I would rather adjust up-down using forward and back motion. If you need more than you can get with that, the pressure from the springy head starts to affect accuracy.
I do think the Javelin head could use a little tweaking. Particularly a stronger magnet for use with the heavier legs. But it serves the purpose and I don’t thing Spartan’s heavier bipod heads would help much given the flex of the legs.
This set up is a compromise that works for me and my hunting. Your situation may be different. I am not denigrating the Springbok either. It’s a brilliant idea and inexpensive (at least the head) for quick and close shots in the conditions I described above. But I don’t think it’s a great choice for my hunting or open country western hunting in general.