For many years vehicles have used lifetime greased joints. The difference between the old-school joints is, the modern non-greasable joints basically use a polished or chrome-plated ball which is pressed into a teflon seat. And a bit of grease during assembly (but not very much). So with the polished ball, teflon and grease (probably synthetic), they've reduced a lot of friction and wear at the ball joint/tie rod end, etc. Whereas in the old days a good greasable joint would have a hardened steel cage around a stud's ball (pressed into the joint). Even with regular greasing you could still have steel-on-steel.
Just putting grease in a non-greaseable joint won't really grease it up. I mean it can't hurt, but ...are you adding a grease which will be compatible to what's in there? If you REALLY want to lubricate the joint .....and most guys don't want to do this because it takes extra work, but if you REALLY want to grease the joint and make them last do this. Remove, the joints and rotate them throughout their entire range (by hand). While doing that, squeeze the boot in ...to work grease inside the boot back into the joint. It's difficult to do it with the ball joints, but if you happen to be doing suspension work, or replacing the wheel bearing/hub, and have the UCA disconnected, that is the time to work grease back into the ball joints. Usually there is a metal ring around the boot which can be carefully removed and new grease added.
What causes a lot of joints to fail is corrosion. The grease can dry out, or not be on the ball portion of the stud. Then the polished ball begins to rust or corrode. Then the corrosion tears up the teflon and you end up with excessive play. So by working the grease back in a little it can extend the life of the joint. This should almost be a routine part of 100k mi service.
Yeah, some driveshafts do have access to the ball/bearing assembly at the double-cardin joint, where present. If not, drizzle a little 90 weight lube on it ..it helps. A guy can also remove the front U-joint (or the rear driveshaft u-joints too), add a daub of 'compatible' grease and squeeze the caps in with a C-clamp. If it's a drilled U-joint it'll force grease into the other caps and lubricate the bearings. But even if it doesn't, rotating the U-joint throughout it's entire range and lubing the outer caps helps too.
All in all, my truck for example, has 173k mi on it, with all original steering joints and U-joints. It drives tight. I've driven trucks with a bit less than 100k mi and the steering on mine felt just as tight. And my H3 has 200k, also drives tight. Sure mine's a highway queen, but non-greasable joints have the potential of lasting a long time. Larger-than-stock tires or water/humid environments will add extra stress on the driveline and joints, so extra attention needs to be paid in these circumstances.