I've heard many complaints about the state of PvZ2. Now, I don't have too much room to talk, since, as my phone has unusually low storage, I'm stuck on Dark Ages Part 1. I don't have your fancy "Magnet-shrooms" or "Tangle Kelps". However, I personally think that it is needlessly difficult.
In the original Plants vs. Zombies, right before or right after encountering a zombie type that's hard to fight, you're given a counter in some way. Fume-shroom to Screen Door Zombies, Hypno-shroom to Dancing Zombies or Football Zombies, Tall-nut to Dolphin Riders, Jalapeno/Spikeweed to Zombonis, Cactus/Blover to Balloon Zombies and Split Pea/Magnet-shroom/Starfruit to Digger Zombies. These plants are also very effective against other types - the Fume-shroom is a master of crowd control, Hypno-shrooms can turn most any zombie to your side, Tall-nuts are powerful barriers that also stop Pole Vaulting Zombies and Pogo Zombies, Jalapenos are deadly instant kills, Spikeweed are nigh-indestructable crowd control, Cacti can act as Peashooters (minus the Torchwood compatibility), Blovers clear fog, Split Peas can be used in effective and unusual strategies, Magnet-shrooms are strong against anything with metal, and Starfruit, when in groups, is extremely powerful.
In PvZ2, counters are gained a while later, or, sometimes, not at all.
Let's cover the counters.
Bloomerangs are early (and efficient) counters to graves and Camel Zombies.
Cabbage-pult are cheap ways to ignore graves entirely.
Iceberg Lettuce are a vital counter to Explorer Zombies, and you get them earlier.
Grave Busters are perfect counters to start-of-level graves and marginally effective counters against Tomb Raisers.
Bonk Choy are powerful counters to Zombie Chickens and Jetpack Zombies.
Kernel-pults are your primary counter to Seagull Zombies.
Spikeweed are counters to Barrel Rollers and Pianists.
Coconut Cannons are typically counters to Imp Cannons and Seagulls, due to their efficiency.
Spring Beans are counters to every land zombie on Pirate Seas, even the Gargantuars.
Spikerocks are even better counters than Spikeweed.
Split Pea are counters to Prospectors.
Lightning Reed are chief counters to Zombie Chickens.
Tall-nuts are counters to Bull Riders and Jetpack Zombies.
Winter Melons are perfect counters to Explorers and Prospectors.
Laser Beans are great counters to graves and Seagull Zombies.
Blover are perfect counters to airborne zombies, primarily Seagulls and Jetpacks.
Citron's Plant Food ability is a counter to machines and Gargantuars.
E.M.Peach are dangerous counters to machinery.
Fume-shrooms are counters to crowds and graves.
Magnet-shrooms are counters to things with metal.
Tangle Kelp are counters to most anything on Big Wave Beach, but are rather inefficient.
As you can see, earlier worlds are jam-packed with counters and pseudo-counters and perfect pairings. Later worlds, especially Dark Ages, are painfully lacking in useful counters. We have counters to zombies we can deal with fairly easily, but not those that truly need it. Prospectors aren't too hard, as are Explorers, both of which have a plethora of counters - even without most of the counters, they're fairly easy to beat. Wizard Zombies and King Zombies are in dire need of counters, as are some of the BWB zombies.
It seems that the Part 1 is moderately sensible in terms of difficulty, and the Part 2 is needlessly hard.