I just bought that slide for a 19 and it just feels wrong. Assembly was easy and it functions properly but it definitely lacks the smoothness and finish of a factory Glock slide. I don't know the composition of either but it also makes a lot more noise if you flick it or bump it into things. I realize that's a silly complaint but it was the first thing I noticed.
Allen has done one slide for me for a Shield RMS and it turned out very nice. I've also had two factory slides worked on by Zeroed In Armory in TX (one with an RMR cut) and they did good work in a timely fashion. https://www.ziarmory.com/
(That RMR is not secured there just sitting, so don't just that gap)