I can't really understand the reason behind all these complaints.

You guys honestly wants Luffy to have a hard time dealing with grunts and a few low rank jail wardens? I mean, Ace is being shipped to be executed and there's also the whole Shichibukai and Admiral preparing to fight White Beard's crew.

I'd imagine it would be easy for Oda to make these guys seem strong so he can stretch this arc out longer and make more money from it. I mean having the heroes spend a ton of time fighting unimportant grunts is the typical shounen stretch. Thankfully he's a better author than that, so we can skip through this nonsense and watch the real action.

Impel Down IS a rather formidable prison.

The most dangerous prisoners and therefore the most likely to escape is kept on the bottom floor deep below sea level. All fruit users are chained with sea stones so they are essentially useless unless they somehow manage to collaborate with other prisoners and have themselves be freed. This isn't like a standard prison where prisoners can go out of their cells and do what they want. Everyone on level 6 is kept locked up 24/7 so the chances of multiple gathering together and planning an escape is unlikely.

Magellan fruit is poison and that by it's nature just scream being able to fight multiple opponents and disable them rather easily. The levels are separated by only one entrance or exit with the wardens being in complete control of the elevator. The prison is surrounded by the calm belt and require the prisoners to commandeer a marine ship to safely escape. The Marine HQ is nearby and is able to send aid if necessary. The chances of someone gathering enough people, planing, escaping, stealing a marine ship and getting away all before the Marine HQ can be notified is extremely low.

The reason why Impel Down is so fearsome is because its very design makes it extremely difficult to escape from.

The only reason why Luffy was successful so far was because he's lucky to have been found by Inazuma. He's also happens to be the son of Dragon or the Okama wouldn't have lift a finger to help him attempt such a crazy rescue. Luffy attempt to rescue Ace is the reason why Jinbei and Crocodile went with him. It's impossible to find another person that is as connected as Luffy to where he can influences all these power people to help his cause. There's also fact that since the Marine main focus is White Beard right now and Ace execution is top priority. Magellan personally escorting Ace out while riots are breaking out is proof of this.

I honestly doubt that a large group of criminals from level 6 might be half as successful as Luffy so far.