ridiculously low improvements of upgrades upon the characters

They do that so new and experienced players can all play together and no one is super overpowered. If you combine your upgrades right you will notice a difference though, I like to use reload speed, extra ammo and more damage together for the best outcome.

I do not agree to the low difference in health points that the health upgrade provides when used though.

but I think that its a very good experience for new players in the sense that this will be motivating to them to make their best to be able to beat the the most experienced

I think they purposely put health upgrade at the end and make it small so it doesn't over powered.

If master level player already strong and has 2 other upgrade then put extra 25 health, it will be almost impossible to kill him.

We can use it for low health character (imp, sun flower, rose, soldier) so enemy need to put ONE or TWO extra bullet to kill you.

That extra bullet could make big difference. When I use health upgrade on imp, I feel like I'm invincible and much harder to die then before.

If you play long enough, you know how many bullets needed to kill an enemy character. Sometimes, you wonder why this enemy is not dying yet. It used to be die by now. That could be because they use that extra health upgrade.

Some variants are fairly poor without the proper upgrades.  Tank commander, iron citron - reload upgrade x2;  any rose - homing; frozen citron - ice; hotrod chomper - digestion x2; alien flower - spore cloud upgrades.  Etcetera.

Then there're the ones that are utterly useless - iron citron, armor chomper - health...  As noted above, a few characters benefit greatly from the health upgrade.

First of all: these upgrade do make a difference. You may not see it but it is there. It also depends on your play style.

Take a Scientist for example. He got 100 HP and 110 with the upgrade. Why does that matter? Because with 110 he can survive a critical hit from a Future Cactus which does 102 damage.

Secondly: it would not be fair to have a bunch of high level players, with many hours of experience in the game, have access to powerful upgrades no newbie can compete with. It would simply ruin it for newcomers when they literally need upgrades to succeed. So what we got is a mix of potential upgrades and fairness. 

There are a few characters that perform better after certain upgrades, like Rose with her homing for example, but even without and the right play style it is possible to succeed. Even more so as a team.

The dev team is focusing the upgrades too heavily on online-multiplayer, which angers me a lot. They should stop for one moment and think about the 'Solo Play' and 'Team VS AI' Aspects of the game; two areas that need the most attention. If this doesn't happen, there will be no fun.

When I went to this game from the first one, it really felt like they slowed EVERYBODY down and made EVERYBODY do less damage.

Then you come to find out that everybody is allowed to have a speed upgrade, damage upgrade and reload/ammo upgrades.

In the first one, your upgrades were set and unlocked through the stickershop. Once unlocked they were there and not interchangeable. There were 3.

The Upgrades are meant to compliment your playstyle. People nitpick the usefulness of Speed Boost over Life Upgrade, but I've come to use both on most healing classes. Just have fun with it :D