I'm not sure to be honest, especially as I don't know what the major traffic flows are (other than straight through in the A55!). In some ways this demonstrates the British approach: "aaaargh! It's got more than four arms! We can't cope, just stick a roundabout in and hope it works". A signal-based junction of some sort might be better (as long as the signal timings are sequenced properly, which generally they aren't), but unfortunately I can't say much more without a load more data.
Fair enough. In case you're curious, the major flows are between the A470 and A55. Most commuting traffic leaving the A55 heads north into Llandudno / Jn on the A470 and A547 (and back out), and there's a LOT of daytrippers added to this during the summer, coming to/from the A55 from the East. There is a reasonable amount of traffic coming northbound on the A470 joining the A55, or crossing over toward Llandudno. Again, seasonal flows include tourists coming and going from the England direction. (The A547 east of J19 is pretty quiet.)
Prior to the remodelling, tailbacks on the A470's flyover from Llandudno were standard in the evening from Spring to Autumn, with traffic seeking to turn left onto the A55 toward Chester. The road only becomes two lanes shortly before the roundabout (much of the flyover has a crawler lane for northbound traffic up the steep slope). Only the left lane was for left turns, so as soon as you had half a dozen cars waiting to do so, no other vehicles could filter past to go south/west.
This flyover now allows traffic from either lane to turn left, with the right lane also serving ahead onto the roundabout (for south/west). It's notable that while three of the major roundabout's entrances are signalled (giving a decent route for pedestrians & bikes to cross), the two entrances from the Llandudno / Jn direction are not. The lights elsewhere are timed to periodically hold up other traffic to let vehicles geto out of Llandudno/Jn, and they're free to enter the roundabout at any other time the coast happens to be clear.
There will also be some tailbacks on the A470 northbound toward J19 at peak times, through the adjacent small roundabout. They're not as bad as the queues of tourists leaving Llandudno in the afternoon, though.
All in all, I think it's a pretty good example of improving the existing road layout, and suiting the main traffic flows. The only other thing I could have wished for when the diggers came out was a filter lane from Llandudno/A470 to the A55 eastbound. This would have been tricky to do without rebuilding/modifying the flyover, and would have required additional land, encroaching on the buildings north-east of the roundabout.