Does the game feel too hard on offensive?

Yes and no. The concept of playing on Offense is daunting, but the maps are fairly well balanced. While there can be a questionable point or two, (like the last point on Zombopolis in some cases) you aren't really gimped for playing on offense - unless your team has far too much dead weight on it.

Bad team composition and skill (or terrible class balance) can make capping a simple point much more challenging than it should be. Nobody likes being matched with dead weight - but it's possible to overcome it. (Although it can be difficult)

The reverse can be true though. Some points are skewed towards offense - like the last point on Seeds of Time and it's overgenerous time extensions for offense.

Time Extensions in general can ruin great defense games as well - nothing's worse than the enemy team barely capping that point in the last few seconds or even well into overtime. The enemy gets a full five minutes or more to derp around with - despite them playing horribly. And if this keeps happening repeatedly despite your team's best efforts, a great defense game can slowly turn into an absolute nightmare.

yes it absolutely does on some maps.  I'm not sure how long you've been playing but it use to be too easy on most maps as attackers but it was changed two patches ago.

Some maps are still pretty fun like moon base Z but others are just way to challenging.

Seeds of Time is very challenging for Zombies, especially the second cap point.

Zomburbia is very tough for plants, especially the second cap point

There are some other misc items that needs tuning as well such as battery at end of zombopopolis and the bootering ram at end of seeds of time.  (both are too difficult for attackers)

Some maps tend to be skewed towards the Offensive team and some are skewed towards the Defensive team, but even then specific points along the map can be individually skewed to be harder or easier to defend so it's a real mix.

As stated elsewhere the ultimate composition of your team affects your chances of success regardless of whether you are Defense or Offense an the skill level of the individual players has a large impact as well along with how those players participate.

I've been in plenty of games where the offensive team didn't play aggressively enough, preferring to stand off and snipe from a distance, only to make a desperate rush to capture the point at practically the last second only to fail. By the same measure I've seen plenty of defensive teams scatter off with the intention of sniping or hunting lone members on the attacking team while leaving the point completely unguarded which leads to it being captured quickly which can lead to a domino effect since the defense just can't get it together. Both are examples of players not playing to the objective.

In cases of a tight, solid defense happening you can have a real run for it as the attackers and in those cases you shouldn't be afraid to change up your character class to try to crack the defense in a different way even if it is just for a few minutes before you switch back to your favorite again can make a difference. There's also a tendency for players up against a solid defense to keep throwing themselves into the meat grinder rather than seek an alternate route or try a different tactic; how many prolonged bloodbaths have some of us seen over an attacking team determined to set up, or continue using, a teleporter even though the defense has that area locked down tight?

Some of it is the map and some of it is what characters are being played, who is playing them and how they are being played.

Agreed I think Zombopolis last stage needs to be tweaked a bit. Either reduce the batterys hp or add 30 secs to the timer. Its a downright bear winning on that one anymore from an offensive standpoint.