I am thinking about buying this game. But a friend told me about a problem with saving he encountered:

He claims that after a save/reload cycle the whole world resets. Destroyed generators are functional again, same for cameras and that the guards respawn.

It that true? Because that would be a reason not to buy the game.....

Yes, world resets on sl.
Is this a reason not to play?
Don't think so. It gives you deeper immersion from 'ohshi~' situations, because it breaks boring cycle 'meh, error- load'.
If you take into account that 3/4 of the team aren't designed for quiet infiltration(although if you buy them right equipment, they can), you should catch bullets here and there.
When I realised that, I completed second sector in less time than two missions from first. Dying on mission also isn't a big deal, thanks to cloning.
So, yeah, if you see 'I lost hours of progress because of bad sl', it's just people which tried to never be spotted. Some of missions are nearly impossible that way. Blasting your way, on the other hand... Bank in 'The Grid' I completed in 2 minutes.

For me it is a reason not to play. I enjoy silent and stealthy infiltration and don't intend to catch a bullet.
And a lengthy preperation, taking out camera and generator and then walzing in unseen - that's my kind of fun.

So if I can't save that, for me it's a game-breaking bug. If I understand your descripton of the game meachnics correctly, there is no error-load cycle because if you screw up, you let your crew die and respawen.
So you rush in, grab what you are going for and run for the exit. You die on the way? Well, just wait till the guards go back to their default jobs and then walk in with the respawned clone?
That's even more boring in my opinion....

Thanks, I think I will skip this game.

Well, if you want perfect unseen walkthrough - game isn't for you.
Silent and stealth infiltration is possible, Infiltrator has a cloak skill, other characters can buy cloak as item.
At second sector you can buy sound grenades to detract attention, and sound dampener, which lets you even blow up grenades without anyone noticing.

Considering that there are assassination missions, you can't complete the game killfree. Although my first assassination went well, sniper rifle with silencer from cloak in the middle of 5 enemies.

Personally, I'm more into bruteforcing, latest mission I completed through exit path - hijacked civilians and ordered them to attack blast door, which could've been opened only from the inside. Then rushed through poison gases, which leaved me at half hp. Then into vent system, and its exit was near target.
Was there more silent way? Bribe 2 guards for 1k each, hack terminals, cameras... twice as long and without explosions. Boooring :)

You understood my description slightly wrong - if you screw up, game makes you feel adrenaline rush 'can I still win despite the error' instead of hundreds of reloads.
If you let everyone die while carrying prototype - you lose that item until you purchase it at black market. VERY high cost of error.
Another cost - degradation of clones. The better unit you hijack, the more stats it has. Commanders give bonus to energy regen (longer cloak). But, after death from 0.6 it degrades to, say, 0.3, after another death it becomes 0. Enemies are on par with your units in stats, and outnumber them. Winning a firefight is nearly impossible, but exiting through shortcuts under fire isn't.
Another costs of the death - xp and money losses.
All in all, zerg rush doesn't work well with deep infiltration.
I used reload... twice, I think - despite 'the grid' being third sector you open, it's easier than second. Was shredded to death in second sector.

I've seen let's play video were the whole squad was hiding behind something without being seen. So a stealth approach is possible without buying something! Skip the bribing and minus the explosions it's four times the fun. So have one guy waiting outside - he never dies. So the item stays where the guy who was carrying it died.
And then let the respawned clone recover it....
Sounds like cheating or using an exploit. As long as the stats are not going below zero that's no cost at all.

That happens even without a save/reload. Guard will go and fix the things you break on their own. You'll always have enough time to take advantage of the hole you've created in their security, but if you come back an hour later they'll have patched it up. If your first team died trying to get the prototype out - why would you think you can just run in and take it?

All that being said: stealth approaches are definitely possible, but hard as hell. The patrol paths are 'fuzzy' - guards will look left/right/behind seemingly at random. The actual patrols are semi-random too. Sometimes it'll be a trio of low level mooks, sometimes it's a high level commander and a posse of drones, or two patrols meeting up with each other looking in every direction simultaneously.

The inability to plan a perfect route is probably why there are several abilities in the game with the explicit purpose of "quietly dispose of guard that just won't look the other way".

Today after work, I actually took my time and visited said friend to play a round. Well, it turns out, if your goon carrying a "packet" you just stole is gunned down, the packet stays where he was shot.
Just wait a minnute and send another guy in to grab it.
On the other hand: If you have equipped said prototype item after you retrieved it and the agent dies - it's gone. And still, if you are behind cover, they just walk right by you and never notice you....