What can ten individual guardsmen do against a marine? They cant even damage him.
I think 1 guard = 1 mag is perfectly fine.
OK, If you give them Righteous Fury, 10 guardsmen are much more powerful than a Mag 10 Horde of Guardsmen (and will take much longer to put down).
Just game-mechanicaly, 1 Mag cannot be 1 person in most instances, because TB + AP is much less than the amount of damage required to take out an individual, or even critically wound him.
I would find examples in the books if I weren't too lazy.
Why would guardsmen HAVE Righteous Fury? Does every guardsman secretly have Touched by the Fates? No? Then they can't RF. Read the core rulebook - it's fairly clear that the only bad guys to have RF are the ones with Touched by the Fates.
And yes, for the most part 1 mag =/= one guy. It depends on what the horde is of, like Adeptus-B said. Grots, obviously 1 Mag can be tens of the things, ditto Rippers. Fire warriors - maybe 2 mag a guy, cause they're kinda badass. CSMs (though why would any GM Horde them, unless the team is using non-errata weapon stats and the players are high rank), 10 mag a guy, maybe more.
In response to the original question - they are very badass-capable. For example, our Librarian killed a Hive Tyrant by attaching a Seismic Escalation Detonator to it. And our Techmarine sniped out a squad of Broadside Battlesuits. And his Servitor kicked a Daemon Prince's ass (which was as funny as hell).
But they get hurt pretty easily. They feel like a glass cannon. Which makes sense, in a way. Weapons technology is almost always a little in advance of defence, and as they're going up against guys who don't mind innovation…