I find that complaints about Champion Testaments tend to come either from new players (understandable) or players who are rarity snobs sitting on 40+ Knight's Testaments. Lower rarities have been buffed so much since launch that they're hardly distinguishable from natural 5*s now, unless you're trying to force one of the dozen or so 3* characters with a bad kit to be meta.
5* characters cost up to three Champion Testaments each - one to break into the final mana circle, one to pick up the Lv3 version of their Skill 1 in that circle, and one to hit Mana Circle 50/Damascus Crystal.
3* and 4* characters only cost up to one Champion Testament, and its the least important one, the one that hits Mana Circle 50/Damascus Crystal. Yes, promoting does cost Eldwater, but we've been positively flooded in that lately - between the backpay from summoning changes (which also roughly doubled Eldwater income per pull going forward), frequent Wyrmprint drops in regular missions, increased event rewards, etc.
Between 3* and 4* rarities, I've promoted 15 of them to 5* rarity, 13 of which still get fielded in my teams, with 6 of those on a maxed Co-Ability. While I have 20 natural 5* characters, only 8 of those get used in my teams, with only 4 having a maxed Co-Ability.
I own Lucretia and Amane. Today I put Xiao Lei to maxed Co-Ability, because I think she's a much more valuable character than her alternatives. When it comes to strictly timed content, the variance of a Crit Rate/Crit Damage buff is very helpful, especially in AI hands. That's not even factoring in how her quick Skill 1 backed by a 20% Skill Damage passive is enough on its own to outdamage her higher rarity peers. Lucretia is still at Mana Circle 40, because I don't believe she's worth the Champion Testaments just to be a worse Xiao Lei. Energy simply isn't a pushed enough mechanic to make her kit any good, with each layer of energy only counting for Skill Damage/Healing Potency+10% for one skill activation per party member, and that's assuming best case scenario where you distribute a multiple of 5 Energy over the course of a fight.