A broken start with a nice finish

CLASS: MG
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This campaign had a real turn around at one point, so im going to grade it separately, one section for the first four maps, one section for the last two maps. 
A lot of the issues are continuous throughout the campaign, so I cannot really name anything in specific. Basically every 10 steps you see the same issues repeat. From floating props to massive holes in geometry or just outright mismatched edges.
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First Four Maps:
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The first four maps had serious design flaws, broken and easily abused nav mesh to the point where I could literally play "the floor is lava" but with zombies.
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Map 1 had an insufferable amount of witches which made expert mode a little more challenging.
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A massive lack of invisible walls as boundaries on the playable area made it very easy to get outside to the edge of the map. On pretty much all four maps.
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Lack of directions on map 1 and a pointless underground tunnel made navigation on an otherwise short map a pain in the ass.
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Floating props, floating geometry, disconnected geometry, massive holes, missing textures, copy/paste houses, forcefully colored cars (use skins instead, or change the color of cars compatible with color changes)
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Slow train event
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No L4D2 Items AT ALL.
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Long, winding, empty maps with lack of supplies and seemingly increased amounts of witches and tanks made expert a pain in the ass.
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Maps are too dark, theres a good dark and a bad dark, this was a bad dark especially on map 1... map 2, 3, and 4 werent as bad.
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Last Two Maps:
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Terrible transition between map 4 and 5.
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Spawn underground on map 5.
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Several eyesores, some guns standing up at weird angles, lack of player leading.
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Visually pleasing compared to the first four maps, the last two dont suffer from the same issues the first four suffered from.
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Levels are more brief, which is nice.
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Levels arent as empty as the first four.
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Exploreable areas with items.
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L4D2 Items.
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Good finale, brief, not too difficult.
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Still a few nav exploits possible, a few "god areas" and a few places where getting out of the map is possible.
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Fun random death hole at the top of the stairs of the finale map, made for some entertainment.
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Overall the last two maps were this campaign's only redeeming quality, and allowed the experience to end on a less than sour note. Rated 2/5, not the worst ive played, but the first four maps need a serious overhaul because there were glaring issues and I swear the person I was playing with died a little inside at the sight of some of them.
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If you put some work into refining the first four maps to coherently match the last two maps, then your campaign would be all the better for it.