At some situations it's more effective to mass produce the exact cheap items needed for your mages\thugs\SCs rather than to shoot for the holy grail artifact or aim to quickly hoard the worthy ones. This approach also saves your research for key spells in other schools and leaves gems for globals/summons.
The shiny gadgets are cool but careful deliberation is required prior to making the investment. The key questions for me are how soon I estimate to return the investment and whether same gems+research time has better use.
Last observation - no unit is invincible. Kit a Seraph with the coolest artifacts (at a steep gem cost) and it can still go down to any number of things: anything from mass seeking arrows to VotD, horror mark spam or custom made ambush. So sometimes it makes sense not to put all the eggs in the same basket
