I don't see why people don't think Darius and Garen should be compared. Both have the sole purpose of solo top, both are melee, and both have a "finisher" ult. The champions are not the same by any means, Garen relies on mobility and Darius relies on CC, Garen has sustain in his kit, Darius has harassment in his.
I do not agree with the statement that Garen's ult doesn't match his kit because that actually dignifies him. What I do agree with is that his ult is too easy to counter and progressively gets weaker as people naturally build MR and gear for it. Low levels is where the ult shines which is why I consider the change to his ult to be a nerf.
Now comparing his ult to Darius, unless you are fighting against a tank that is building all HP and no MR Darius's ult will out damage Garen's. Garen's ult has a higher damage, but not high enough to make up for % loss from MR and not to mention the reset on Darius's.
As for people escaping Garen, I find this to be more of a problem now after his slow removal was put onto his initiation ability. This makes Garen going in for a gank very easy to stop with slow and takes away his ability to have an exit strategy. Most of Garen's problems go away with a Frozen Mallet, I also like to build a Phantom Dancer for faster gap closer and building Boots of Swiftness first.
I would love to see Garen tweaked in a few ways such as slow removal put back on his spin to make it a versatile ability again and make some way to build magic pen based on % maybe 10 magic pen per 100 armor and MR to give Tank Garen more damage.