You're absolutely correct: why can't more recordings get these details right? First of all, I really think it would greatly help if the conductor would rehearse the ending with just the percussion alone. At the spot where the trumpets (with the horns?) are sounding just the tonic and dominant notes - the final permutation of the "Resurrection" motif - the three bells and two tam-tams have a rather akwardly written polyphony: alternating salvos between each of them. It's akward in that the conductor is beating in a slow "2" beat pattern (two beats to the measure), while the percussion have each measure there divided into three sub-beats on their page. Mahler frequently asks them to time their salvos on the second and third sub-beats. Given the fact that it's difficult to see a page while strike a gong or bell (particularly the tall tubular chimes), I'm sure that many percussionists end up faking the part somewhat; or try to play it by memory. It's also difficult and visually distracting for the conductor to attempt cueing each of those salvos. Again, the rythm and timing of them is more difficult than you might suspect. So really, in a sense, it's no different than having to work out many of the more difficult percussion passages in the "Rite Of Spring". What's truly necessary is for each percussionist to hear and know, when the other four percussionists play their salvos as well. In short, they need to be made confident of what they're doing.
Now, once the rhythm is correct, there's the issue of getting the balances correct. I've often thought that that very final passage should be recorded with the percussion in the middle of the orchestra (obviously, I'm talking about a "studio" recording). The timpani would stay behind, along with the brass. But the bells and gongs could be placed right, smack in the middle of the orchestra. That way, they could see the conductor much better, and the percussion could be prominant. If that sounds too bizarre, remember that the strings and woodwinds are just holding long chords at that point. Just for that final passage, you seat the orchestra around those bells and gongs. Obviously, you wouldn't want the mikes close-up on them. But that's rather the point: get the balances correct without sounding as though the percussion were miked separately.
The other option could be to record the percussion separately (gongs and bells) - so that the balances are absolutely perfect - and then dub that into the recording. With this approach, you'd record everybody else without the gongs and bells playing along with them. Artificial? Yes, but the rhythms and balances would be spot on.
Barry