I didn't know there was a subway, so I looked at Streetview and found both ends of it. I must have passed its western end hundreds of times.
Most subways are pretty awful
IME▸ – probably not actually as dangerous as they feel, but certainly as depressing – but this one seems to be largely open, so hopefully it's not so bad. Nevertheless, I can pedantically claim I was technically correct in saying pedestrian access from one side of the A38 to the other is awful, as the subway only connects to one side of the A38.
I can't think of any reason to cross the A38 there that would no be better met by using the subway, but having said that the access to the subway from the bus stop on the A38 is via a flight of steps, which could do with improving.
The A38 infrastructure between Filton and the M5 is very much 'of its time' - which is to say massively overengineered, and totally unfriendly to non-motorists. It's interesting to compare the huge flyover at Gypsy Patch Lane with the new junction just to its north, which presumably takes about the same amount of traffic or more, without all the civils.