TLDR: Calculate the effectiveness of given itemizations on a given champion, within a given situational context. Then being able to use this data to suggest proven effective items for a given context.
In League, itemization is obviously a skill that needs to be learned in order to master the game. As League stands now, I think there’s a lot of difficulty in seeing how important choosing items that mesh well with a Champion, and also items that adapt well to a situation. Websites like Champion.gg, even MobaFire, ultimately offer static builds or superficial suggestions irrespective of a given matches context, or update to game patches. “Of the 200k matches we analyzed with Gangplank, those in which GP won, GP built such and such”. This is does not provide context and reinforces the idea of monotonic and rigid builds for champions, rather than adapting to the situation of every match. There is a lack of insightful information to answer the important question of why GP was successful with those items in those situations and furthermore only serves to lower the usefulness of such websites in the public’s eye.
You see builds like full AP Ryze with no mana, are these builds wrong? Who’s to say, variance is a quality that you Riot, go to great lengths to ensure remains viable. However, there are simply items that resonate more strongly with champions, in the Ryze example, the player is obviously missing out on arguably half of Ryze’s scaling. Even the pro’s seem to sometimes make itemization mistakes. In the recent LCS split, we saw a Moakai top, up against a team mainly consisting of Magic damage, however this Moakai chose to build some armor (likely going for a Glacial Shroud, for the mana and CDR) which ultimately and not surprisingly proved ineffective.
Choosing the right items is hard (perhaps some of what contributes to the difficulty is how hard it is to see how each item contributes to a game). Take for example a game on Summoner’s Rift with Anivia, say she first builds a tear. Then say, wants to decide between a Rod-of-ages or an Athens? Which and what component to build from (Catalyst, Chalice, etc.)? Stat-wise ROA has favorable stats for an early Anivia (she has low base HP) however, why and in what situations makes the Athens more favorable?
Superficially from the above, if we go for a ROA by building into a Catalyst first. We would have a stat preference model (SPM) something like: 
If a player chooses an Athens over a ROA and is successful in their endeavors (say Laning and ultimately significantly contributing to winning the game), they alter our preference model akin to something resembling: 
Or some more simpler questions: Diana (mid lane): starting towards Zhonyas or an Abyssal, or a Stringer, or a Deathcap, etc. AD Kog: BotRK or Rage Blade? Or Brand vs Swain, Brand starting Morello or Athens? These help to illuminate how ineffective current, automated build suggestion sites are.