I just noticed that your summons strength depends on the difficulty level you play on. E.g. if I play on novice my summoned creature goes down in 3 hits by me, but if I play on master it lasts for about 30 strikes and kills me (and my opponents) really fast.

This makes it really overpowered on master difficulty. Should I still use it?

Hmm

Actually, difficulty only affects the damage YOU deal and recieve. Summons strength are unafected by everything else I'm pretty sure.

Im not quite sure because summoned creatures do take damage and deal damage at a different ratio to other npcs on different difficulties

Novice: 2x Damage Dealt and 0.5x Damage Recieved
Apprentice: 1.5x and 0.75x
Adept: 1x and 1x
Expert: 0.75x and 1.5x
Master: 0.5x and 2x

So my summons don't get stronger against enemies? Because I noticed that I struggle really hard on master with my level 5 archer/melee hybrid, but when I get my summon out it can easily deal with 5-6 bandits at once.

They stay just as strong against everyone on all difficulties. That is why they have an easier time on harder difficulties. As Virtuosus posted difficulty affects your damage dealt and recieved. Your followers aren't affected by difficulty either, only you. This is actually one of the few legitamate ways to play Oblivion on max difficulty without wacking enemies 100 times each to kill them. Your summones had a much easier time.

Is there any mod that fixes this problem? I play on master because I want the game to be a challenge, but I love conjuration and need it for my playstyle.

It's only doing more damage to you, not the other npcs. So it's not really a problem. Also, not all bandits are the same.You might just be having difficulty because you're an archer/melee hybrid with conjuration, and you're leveling too many skills at once.

The problem is that my summons are overpowered compared to myself. When I play with summons on master its basically like playing on adept.

Problem? It's a game mechanic, that's how it's supposed to work. There is no problem.

I don't understand so are they not effected by difficulty or they are?

As far as I got it, the only one affected is the player character. Followers and summoned creatures are not.

The difficulty slider only affects the damage you take and receive, so at adept you are taking dealing 1x damage to your summon and at master level you are dealing 1/2x damage to your summon (and every other enemy).

The damage your summon deals to enemies and the damage that enemies deal to your summon are the same regardless of difficulty.