You could argue which is the proper stuff or not, for a pukka job steel tubing would be used or most other metals first. The pu type is sold on grounds its easy to install with minimal tools. True price has dropped considerably in 7 years, 16mm pu tubing on par with copper 15mm tube but sting on all the extras needed. ie without shopping around 25 pack 15mm copper elbows way less than a fiver whereas if can find the 16mm pu type theyre close on a £5 each not to mention all the pu to various thread terminations needed.
When done my copper install I did look in to the pu and a starter kit back then from £120 . To do what had to do would have cost way in excess of £360 and be pretty basic with no built in U bends/traps as against copper install around £70 which included 10 off 3M lengths of tube £33. Just the feed from a splitter to bead blaster excluding any other area 4M tube £8, 7 elbows £35, 2 end fittings to thread of choice £10 and stop cock £10 = £63 at current pricing. Copper works out for same install £15.4 for a neater better job and easy to work with.
Another point the air you breathe has a certain ratio of water makes no odds squirting it back to atmosphere that's where it came from.