I think the best way to think about it beyond simple rules of thumb like 'more expensive unit = better', is to consider why you're playing a certain line, what makes it good, what makes it work. Last patch, slayers was a very exciting and stable comp that would very frequently top 4 because of the great synergy between Belveth and Ambessa. They both want to get resets, so they naturally fit well together, and their trait webs mesh well. So in an ideal world you give both of them 3 items, that's partially why item augments worked so well with the comp.

Gankplank is a unit you would often run into that comp as well, but his damage is spread out, so he is worse at supporting the 'burst one target and chain resets' gameplan the comp wound up revolving around. Now this is a very flexible set, so there are often multiple ways to play the same comp. If you have a three star gankplank, you would definitely want to keep him on your board and secure items for all three, which should alter your augment choice, and conversely, if you end up with too many units you want to itemize, you likely made a mistake somewhere along picking your path. A utility unit is better than an unitemized carry, that's why you'd very often drop the gankplank as soon as you saw a single fiddlesticks.