At what point is it better not to upgrade vets?

[PandaChef]

As the title says. I have just got a lvl 5 katana samurai, with 2 in melee attack, 1 in morale, 1 in defence and 1 in charge with another 1 in clan upgrades for melee attack. It costs 1020 koku which is pretty expensive, should i keep upgrading or what? What is the point where you stop upgrading as it becomes so expensive you would be better off using the money elsewhere?

[Salvator]

I would have stopped after the fourth upgrade, 2 in attack, 1 in defence and 1 in morale is most of the times enough.
But it's more on how you use your Katana Samurai. Are the Katana Samurai the ones who win for you the battle? Or are they more the ones who buy you time for your cavalry to deal the coup de grace?
If your core lies in Kat sams I'd spend extra points into melee attack, if not leave it at the aforementioned 4. But as I said it is on how you use them.
I for example have a Kat sam with 4 points into attack, 2 into defence, 2 in morale and 1 in armour. This is my battlefield tank and I do everything to engage him in combat safely with the given factor that my enemies units are all bunched up.

[TheCrazyCat]

1st 2 upgrades. 1-2 attack thats it.

[Szlachta]

Depends on the vet. A Marine? Rank 5, for 150 range. A White/Red/Black Bear? Depends. 2 accuracy/2 reload, unless you want to get them to 9 for 150 range. On medium, or large funds, I usually bring 1-2 marines, and 2-3 150 white bears. Cavalry wise, no vets. Melees get owned by guns, so I (you) don't even need to bring them.

[CptAustus]

At the point the upgrades arent worth the extra cost.

[Knowing Crow]

I really only upgrade with a clear goal in mind. +Range on Tosa Riflemen doesn't cost 120, it costs 520. The fact that they have improved stats is nice but the real point is that they have 175 range. My Yari Ki have 100 points into +Charge, which has the specific purpose of insuring that they win against opposing cavalry charges.

Placing random points to make a unit marginally better is ultimately a waste of points. What exactly do you get when you upgrade them? What amazing new things can they do, what are they capable of that vanilla Katana are not, and is it worth that extra cost? Not much, actually. They will always be able to cut up enemy line infantry, and if you improve their stats in that regard it won't help that much as they are already very capable of it before hand. Ultimately your best bet is keeping them vanilla because vetting them doesn't give you anything.

[perablenta]

What you need to learn is which upgrades are cost effective and which are not. And what retainers to use for upgrades that are too expansive to put on units.

Most, if not all clan upgrades are NOT cost effective. About the only one that you will find of any use for Shogun 2 units, is the replenish upgrade for hero units that you like to use. That is if you use any of those for your standard build.

Morale, fatigue resistance, armor, speed are close to useless and very cost ineffective . Charge is only good for cavalry and no-dachi, and the latter are cost effective as an upgrade but a hit and miss as a tactic.

For those upgrades you need to use retainers because, for example a retainer that adds +1 to morale to all veterans and +5% cost to monks, for an army that doesn't use bow monks, can save you from 400 to 700 koku. Because it affects most of your units.

That is why retainers that add bonus attack or defance for only one type of units are to be avoided and corresponding upgrades given to veteran units.

This way you can save retainer slots for retainers that affect your entire army, which are very cost effective, while you can use upgrades to buff up specific units.

Also you need to remember that a too expensive veteran unit can become a burden rather then a asset because lossing it on the battlefield, or simply being bogged down by a 350 koku yari ash can hurt you a lot. You didn't just lose a 1000+ koku unit, you also gave your enemy a 1000+ koku advantage.