Many say that
Miracle Day was overly long and drawn out, and they're right. What it was trying to do was replicate the success of
Children of Earth, but no US Broadcaster would have been interested in a series lasting just five episodes. Nor were we ever going to be lucky enough again to get 'event TV' where they air one episode a night for a single week, akin to Children of Earth - not that such a stunt would have probably worked twice. It was stuck in the form of a ten part, weekly series and yet still the execs seemed determined to try and copy Children of Earth with a moral dilemma serial sci-fi. It backfired big time.
The US filming added absolutely nothing... the series aired in 2011, the same as Doctor Who: Series 6 which featured episodes filmed in Utah. While I feel both shows could have done more with their location filming, Miracle Day was an abysmally poor effort and could have just as easily have worked having been set in the UK. All it did was undermine the Welsh and English setting.
The concept behind the series was an interesting one, but not one explored particularly well. It completely rewrote the logic behind Jack's immortality to fit its own developments, and the characters weren't captivating enough to warrant proper moral debate. I remember in Children of Earth: Day Four there was a scene in the Cabinet Room with a whole bunch of politicians trying to decide which 10% of children would be given up to the aliens. It was a difficult, tense but clever piece of television - a bunch of characters we'd never seen before, and didn't even know all the names of... but it was gripping! It lasted a good ten minutes and really made the viewer think. Miracle Day didn't ever have such coherency or intelligence... any time it had the chance to explore strong moral themes it cut them short to deviate off on a sub-plot... towards the end of the series we had a whole episode set in the 40's at which point it was clear the idea (and the series) was running out of steam.
I really struggle to find much good to say about Miracle Day. It was slow, it was dull, and the Americanism's crept in far too much... the violence, the gore, the sex. Not only had Torchwood wisely distanced itself from these 'adult' notions as it had gone on over the years, Torchwood itself was a show that always enjoyed spoofing the identikit US shows of the genre - the type that had exotic cars and sweeping shots of Los Angeles... that's why it had a Land Rover and sweeping shots of Cardiff all the time!

But now the show is in LA, and it's become the thing it was deriding. There were a few impressive action sequences in the episodes, and a few of the American actors did a decent job with limited material but otherwise it was a ten-part bore-fest which completely took a u-turn on the maturity and innovation that Torchwood had taken on board.