Yea, admittedly, this is a case of "I have a box of 10 Reivers I picked up for Kill-Team", and as stated, I'm pretty much on the "Primaris-Only" train for future Astartes purchases and army building (unless I can find some useful place to stick what Firstborn marine models I still have that I can still field, both from a "having to fix them up from early years of bad painting, or missing parts" and a "Are these guys even still worth the Force Organization slot/points?" perspective. And well, DW Black and Silver is looking tempting for that if I want to keep my classic Ultramarines Firstborn rolling).
My main concern with Knife-Reivers still is that they're a unit which should probably still be deployed by mid-game, specifically to chase ObSec units off the board, but they really don't ramping up until Turn 3-4 (and as you pointed out, playing a Ranged combat chapter and flipping to Assault immediately is a tough strategy to play around. More so when relying on Infiltrators for the time being as my troop choices, I need Tactical Doctrine to give a bit more bite to their Marksman Carbines.
I guess the better way to phrase it in my case is this: For Ultramarines, should I build Reivers All Knives, All Carbines, 5 with knives, 5 with Carbines? And also, which mobility option should I pick with each?
Building Infiltrators from the Shadowspear box. Which my strategy for them is definitely pretty basic, Stick'em on objectives for early points, try to keep'em alive with cover and Smokescreen, +Omni-scramblers, but take advantage of Ultramarines ability to make fighting retreats/tactical withdraws as needed. Especially since Ultramarines appear to be built around mid-game power with late game clean-up.
My main concern right now, is the obvious gaps. No Anti-Vehicle. Relying on two units that can only field as units of 3 models for dealing with big threats each leaves me a bit wanting in the ranged department. Reivers are good for shutting down ObSec and chasing weakened Troops off the board, but cleaning up other units becomes a more difficult proposition. And Phobos HQ tend to be prime Melee charge targets to your opponents (with the exception of the Librarian)