Cu Alter worth it?

5* servants are always worth it. Cu fulfills a much more rounded role than Jeanne Alter too. Cu's is more more about his survivability, and to some extent, his team's survivability with his first skill. While he may not be your main DPS since he has lower burst than Jalter, he can definitely be a last-stand clutch with his per-hit dodges, guts skill and defense stacking capabilities. He still hits like a truck, despite his lack of steroid skills.

He might even be good for a more offensive Jalter team, since he does make an OK amount of stars with a Quick-involved brave chain, and has an even lower star-absorption than Jalter herself.

In my opinion, yes, I would use the mats to max him out. I also have Jalter and she is certainly one of the best damage dealers and is more serviceable thanks to her Avenger class. With Cu though, you have the advantage of the Beserker class and the extra damage dealt. For a beserker, he is also pretty survivable thanks to his on demand doge that isn’t just single turn. He can also weaken enemy attack and Crit rate. I’ve certainly found times were his damage output, especially on his NP, out does Jalter against harder boss fights and events.

Jalter is certainly a better Buster Crit servant, but thanks to his class advantage Cu alter can certainly match and in some cases exceed Jalters output as just a single target buster damage dealer. Plus he has high hit counts which allows him to build up his NP better than most in his class. I don’t think you’ll be disappointed if you dump the mats into him.