Did they ever get rid of that crap where the enemies would level up with you? That was very annoying, made leveling useless.
They don't actually level as high as the player though. The enemies reach a level cap well before the player does, and if you choose your weapons carefully and upgrade and maintain them well, the enemy leveling that does occur whilst playing is plenty manageable for the most part.
I found in most cases on the official forum of the game when players complained about enemy difficulty, they were usually not timing their use of Fury very well, or even using it at all. They also weren't planning safe spots along their route to hack and slash from or just build stamina back up. Rooftops, car roofs, or even dumpsters can be life savers. Using vehicles to get around when you can and run over zombies with them is quite effective too.
My biggest gripes about the game, and many felt the same, was that chest scavenging was somewhat pointless due to the weapons you find in them being less than you want, even with loot skill perks, the weapons degraded too fast, and there were a couple places where the infected waves came too frequently, without chance to relocate or prepare. The 3 Heads of Cerberus mission was one of the hardest due to the infected, yet they rated it only Medium difficulty.
On Riptide, I'm not expecting them to redeem themselves story wise, but at least the gameplay looks more inventive. They needed missions like this in the first one, where you have to strategize a defense system and maintain it.