I've only played during open and closed beta, and maybe it's because only players who cared about L4D type of games played during then, but most pubs wasn't that bad on medium, and we made it to the end a good 70% of the time. The other 30% tend to be someone had to leave because of real life, and the bots that replace them couldn't even preform at the baseline of a l4d2 vanilla bot.(Yes the bots during beta was that fucking bad).
If you got tied down by a spitter or what ever the fuck that version of the SI is called, and needed to be meleed out, you are basically dead. Bots couldn't even fucking free you.

They would just stand around you, as you slowly lost HP, and stare. It was basically like they was damning you for playing, and letting them exist.
But aside from that, rest is all spot on for how the beta was for "medium" setting boiling down to. Send a million special infected with HP pools bigger then tanks from L4D. My biggest fucking gripe from beta was how boring Medium got after a bit. All the cool corruption cards are tied to nightmare, and so if you wanted to shake up the gameplay, you had to play it, just to see some of the more neat cards tht change how you have to play.
Acid zombies, timed hordes, more sleeper spawns, levels having no power, ect.