Like everyone else is saying 1st Masteries then after that save them for harder content energy refills are a good cost for only 30 units but wouldn't do it all the time otherwise your unit stash will disappear real quick.
I first used my units to build a full suicides mastery and then used them to slowly become uncollected. Now I am using them to 100% act 5. I once used 2k units to buy 10 Grandmaster crystals and I regreted it ever since. So, as many have said: masteries and hard content.
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