Would be a lot easier to say something about the SFX from a video. Now it's mostly random stuff happening here and there and as a listener I don't know what to expect. But nevertheless they sound quite good and appropriate for their genre.
Your 8-bit stuff is good. Sci-fi is also OK but every sound is somewhat similar to each other as if they're filtered with the same preset.
Piano/drum theme depends on how long the player would stay on the menu screen. It gets repetitive at some point so you could try to grow it up a little bit later on.
Zombie stuff sounds quite weird, like cows or moose. I think you've just slowed and pitched down your sounds and therefore they lack the high end and anything that gives them interest. You can try to maintain (mix in) some of the original sound to give it a little crisper feel and add some gargles/breath/etc. at normal speed/pitch. Use layers and saturate to glue together.
In most cases any sound effect needs its highs, mids and lows to sound realistic. They can be different layers to be more easily controlled. So if you lose your highs or lows when altering speed/pitch be sure to bring it back somehow. Of course there are exceptions like really small creatures that don't have any low end in their sounds.
I'm happy to re-evaluate your sounds if you decide to redo some of them at some point :)