I get quickly tired of mystery boxes, especially when the mystery doesn't actually advance the plot, and just fills in backstory (two recent examples, Rey's parentage, and the reveal in the Doctor Who finale). Things the audience doesn't know yet can be great, but it's hard to pull off well.
I'm not sure I'd classify Rey's parentage as a mystery box exactly, that's more just incoherent storytelling topped with some good old fashioned lack or originality. Whether it was the intent or not, I rather thought TFA answered the question quite succinctly:
"whoever you're waiting for...they're never coming back..."
Then TLJ for some reason felt compelled to underline said point and circle it in big red marker pen:
"They were filthy junk traders. Sold you off for drinking money. They're dead in a pauper's grave in the Jakku desert. You come from nothing. You're nothing." You know, just in case anyone wasn't paying attention the first time.
But then TRoS doubled back on it and was all like:
"No, actually, it was Palpatine all along! Oh no wait, that's the explanation for who Snoke is! I know, it was REALLY Palpatine all along! Huh? What do you mean that's our explanation for why Ben fell to the Dark Side? Oh hey, I have a much better idea about Rey's family...it was Palpatine all along!"
There's no mystery box here, just blather.
As for the Timeless Child...honestly it didn't especially bother me, but then I was rather 'meh' and the last few seasons anyway, so not massively invested.
That said, it was a damn sight more interesting than "the hybrid". DW just doesn't seem to be able to have really compelling arc twists. I mean it worked once rather well with Bad Wolf, but ever since then it's all been very hit and miss....mostly miss.
As much as people rip on Lost, I'm starting to think it might have actually done a better job of giving answers in the end.
Sort of a quantity over quality argument though, isn't it? I mean is it better to have no answers, or tons of answers that are flaming hot garbage?
Both BSG and Lost and the persistent problem of introducing new "mysteries" without ever solving or significantly advancing the old ones. The difference is that BSG just ditched the old threads and kept moving forwards, whereas Lost spent the last season or two frantically running around desperately attempting to knot all the dangling threads together until what they were left with was a rambling, incoherent gordian knot of a narrative that could *never* have a satisfactory solution.
On top of that, Lost committed one of the cardinal sins of constructing a mystery: "thou shalt not bottle out and change course on the central premise mid-flow just because the audience figured out the island is purgatory almost straight out of the gate...then spend several seasons reversing course only to circle back and be like: NOW they're in purgatory for really realsies!"
BSG never ran into that problem because they the audience could never guess what even the show runners didn't know going in!