Nice review - I'm very happy with my IYONIX pc too and thanks to Aemulor Publisher and the odd other 26 bit app performs well.
As for missing things: Well at the time being I'm missing some things like CDROMFS (Select CDFS is not as good!), Win95FS and LanMan98, ImageFS and a 32 bit Browse (yes, the old Acorn thing) would be nice since on my ex-Risc PC I used that browser in 80+% of the cases due to stability and speed and I still use it on my IYONIX with Aemulor. Unfortunately WSS seems to have kind of vanished - what a pity!
As for Select 3 features: Well it does address the odd part I miss to a certain extent (Joilet for CDFS and ImageFileConvert to replace ImageFS) but when I look at the Select features as published by RISCOS Ltd the only nice things that do come to my mind which are not in RISC OS 5 are around the ImageFileConvert like e.g. Thumbnails in the filer. The other Select 1-3 features are either in RISC OS 5 (like DHCP) or available as separate application or not of real interest to me (like rounded buttons, delete protection for !Boot...).
Oops, I missed one: Whell mouse support but no surprise I missed it, since it never worked on Select for me since with PS2Pipe it dot't work but I found that one out the hard way since ROL never mentioned that before offering Select 2.
But the main goodie about RISC OS 5 is that it is 32 bit and offers HAL - that is finally solves the two main initial missions of ROL and thus RISC OS can move forward to new hardware.