It depends a lot on what you buy.

I buy more specialized stuff (or else on Amazon, which no one other than Hawaiian Arilines supports through a shopping portal), and often there's only one (if not zero) retailers available at a shopping portal (at only one major airline at a time), and then half the time it turns out not to work.

Just as an example: I need specialized "advanced amateur" photo gear (filters from a particular manufacturer, particular types of 35mm film, etc), and only a handful of big online photo retailers stock this stuff "deeply", like B&H and Adorama and (to a lesser degree) Calumet and Freestyle. Delta Shopping used to have B&H a year or two ago, and it worked ok for me then, but the next time I went to use it, they'd dropped it. Either AA or UA (or both) Shopping had Calumet for a while, and it worked half and half for me, but then they dropped it. Delta Shopping (as well as AA and/or UA) picked up Adorama, but it hasn't worked for me, on several tries, so I just gave up and am back to going to Adorama's site directly.

It seems that every time I want to buy something, it's either easy to find on Amazon (and I want to use it because I have Amazon Prime free two-day shipping on most stuff, plus it keeps records of my purchases "for all time"), or I need a specialized store that's not likely to be on a shopping portal (or else the mileage earning doesn't work reliably). If I want a book that's not on Amazon, it's at an esoteric site that isn't in any shopping portal. If I want a CD that's not on Amazon, it's on someplace specialized like CDBaby or Dusty Groove that isn't on any shopping portal. (And clothes/shoes/etc, I don't want to them online because then they don't fit.)

So I don't have enough non-Amazon spend in enough categories that can be easily found on shopping portals. (And meanwhile, no one online shopping retailer can take the place of Amazon's depth and breadth. And I find it too annoying to have to search for something on dozens upon dozens of other sites, when the one Amazon site almost always has it.)

Of course, not everyone has my buying patterns. But I'm just saying, it depends on what you buy, and how "tied into" existing online retailers you are, as to how well shpping portals can work for you.

For using them occasionally (say, for a promo, or to keep an account alive), they can fine, except in those cases you have to be very careful which retailer you choose, since so many may not post automatically (or at least in time). But beyond that, you may or may not find it to be more trouble than its worth, compared to churning credit cards, using dining programs, etc, if those are something you can use instead. (I find it much simpler to use dining programs than shopping portals, myself, in part because the dining miles actually post very reliably while the shopping miles not so much.)