Zandronum

Is it a bad idea to do multi story levels with 3D sloped floor? or is it better to just do it the old fashion way?

any glitches or issues i might run into?

not saying bi/two floored, i mean three or more

To be bluntly honest, don't do it. I am saying this not because it's not impossible, but because you'll be in a living nightmare of not only of the 2.5D environment but also (G)ZDoom's limitations. Imagine if you will, seeing a 256x256x2048 area filled with an abundance of lines, you'll be utterly confused as to what the lines represents and hope for the best that there's no conflicting lines. I feel as if I should post a screen shot of a 'worst case' scenario, but I am currently on my tablet and I don't have time to construct a five layer map using an lunacy amount of sector references.

You can probably and successfully construct a map with two floors (stacked) - I know, because I also accomplished this myself with my TGRDM3 project - but exceeding anymore is going to be overly complexed and extremely hard to maintain. If you are really determined to accomplish multi-levels that exceeds over two layers, I would have to recommend on using Model Objects instead -- though the cost of using this is far worse on GZDoom's rendering engine.

In conclusion, I recommend on sticking to just two layers at most. Any layers beyond two, you will have a tough time in maintaining and will spend more time on constructing the multi-layer areas than what you normally would with just one default layer.

I'd have to agree with Tiger here. I once attempted to do multi story building just to see what i could do with 3d floors. (using no sloped 3d floors) I encountered a bug where i would get randomly blocked by invisible walls varying in height. This happened in places where no linedefs were even drawn. I'm fairly certain this happened with (g)zdoom too.

If you must use them, do. But don't go overly complex with them or have several stories.