The ending would have been fairly decent had it simply concluded before the epilogue, with Yuki isolated in despair in an endless void devoid of light or companionship while in the "third world" the parallel incarnations of himself and all of his previous associates live happy and comparatively ideal lives. It would be a fittingly grim or bittersweet ending to a reasonably morbid series, especially as it would find the protagonist ironically returned to the same emotional and social situation in which he began the series (an eternal and utterly isolated bystander).
Of course while it is a relatively fitting ending it's also intrinsically going to be unpleasant for anyone who liked Yuno, the central romance, etc., something of which the author may have been conscious when he decided to catastrophically reverse everything about it in the last 5-10 pages through recourse to an enormous ham-handed cop-out. By trying to awkwardly fuse a somewhat dark ending with an absurdly ideal one, the end result is ultimately just an insult to the series as a whole (which despite some rather substantial heterogeneity in narrative quality still had some remarkably strong features).
In particular, the suffering and development that every character had to actually go through to reach the end is more or less entirely destroyed and trivialized (alongside all of the emotional buildup in the finale itself), as the new incarnations of all of the characters have (comparatively) perfect lives (apart from 3rd world Minene I guess) and the series tries to casually hand-wave away the demise of Yuno (significant, at least, to me in that I found Yuno to be the sole driving force of the entire series) by transferring all of her memories to her ideal-world self. It's just a sloppy, awkward ending that invokes a "power of love" deus ex machina in the worst way possible and makes the whole series feel pretty hollow.
I contest that there was no other means to end the series without killing off one of the two leads. For instance, Yuno and Yuki could, assuming that they could exert sufficient influence on either of three parties capable of arranging it for them, have continued to simply timehop between doomed timelines, living out their lives between and across worlds. Alternatively, they could have simply traveled far enough back in time to live out a normal or desirable life without risk of coming into conflict with the Game, the end of the universe, or their matured parallel selves.
Given that Yuno was able to abandon the 1st world to live through the 2nd without any apparent negative consequences for herself indicates that it shouldn't be too much of an issue for the pair of them to abandon the second world to live through the third (or a succession of parallel worlds) for an extended period. Honestly though, the route to the "ideal" ending provided was so hamhanded and awkward that just about anything else would've come off better- I mean, what in principal is to stop the pair of them from immediately invoking Deus in the Third World and attempting to negotiate (or somehow usurping, as I guess Yuno would still technically be a god of one world) their way to divinity or release (given that Deus is apparently capable of both power-transfer to individuals and giving one diary-identity to multiple owners as with #7) well before the game event starts?
I mean really, just about any alternative would be more interesting, plausible, and enjoyable than the ludicrous split-second narrative reversal and tone-change that happened at the end of the manga, with Deus spontaneously deciding not to settle the affair of his succession with a massively traumatizing bloodsport for once and virtually everything getting tied up with a neat little bow without any apparent initiative or effort from the protagonists.
The number of plot holes and characterization inconsistencies in the series coupled with how massively jarring and arbitrary the true ending was means that even an alternative quite inconsistent with how the constraints of the Game had previously been presented would still basically end up being reasonable. At the very least it could have been Yuki and not Deus/Yuno who initiated the merger of parallel universes at the end, so that at least he could have been shown to have finally accomplished something (I guess he managed something like this when he broke out of the shell, but the immediate outcome of that was basically the same as if he had killed Yuno when she offered to die for him four volumes ago) through his own force of will and initiative rather than relying upon his allies and benefactors.