Dear god, they're generally terrible.
I liked the one to get Bros, but that was pretty much the only good one.
It was the only one that felt at all intuitive. You find yourself in this mysterious tomb that was built in memory of some king that no one knows the name of (I did have a good long rant about this previously) as plot related busy work, and it's up to you to explore.
Yeah, it is busy work, but it's logical busy work so it gets a free pass.
The item you're looking for is right in the entrance way of the tomb (which makes you wonder how incompetant the last person sent on this job was), thus leaving you with the option to explore the tomb but not shoving it down your throat.
It's up to your own interest and curiosity to lead you into the side quest and you get a really good reward for completing it.
This is basically the reason why it's the best. It's integrated into both the story and the world in a natural fashion. There's none of this faffing about with magazines and finding random items, or pressing the action button when the event trigger is completely invisible, or even investigating some random piece of the background with a specific character in your party.
The tomb is a location of interest and characters in the game actually talk about it.
Heck, it's a handy place to go to as it has draw points and monsters you can level up the newer members of your party with.
One of the other reasons it's such a shining example of a side quest is because it requires thought to get through. There are plaques that give you directions, or you can use your own ingenuity to find your way through.
Most of the side quests don't have that.
The other reason it's a great side quest is because you aren't forced to wait until the last third or so of the game because it doesn't require the Ragnarok.
You're cut off from most of the side quests in the game for most of the game for this exact reason. It's ridiculous.
Honestly, most of the side quests in this game don't come across as having any thought put into them at all. The Bros. side quest is the only one that doesn't feel like an after thought. If anything, the way the game forces you to go to the tomb feels like something of an after thought.
I'm not saying that it couldn't be improved, it really could, but as it stands it's a pretty solid side quest. Basically, it's a good introductory side quest. Unfortunately there's not really anything for it to introduce you to.
All the other side quests are far, far weaker, so many of them require Ragnarok.
It's not the globe hopping aspects of the side quests that make you need the Ragnarok, the Garden is a slow but effective way of getting around, it's the fact that every ne of these damn globe trotting ones requires you to fly to elevations that the Garden cannot reach.
They were specifically designed this way, which is ridiculous because the rewards really aren't good enough to warrant the amount of time you waste running around doing them, nor is the challenge enough to be entertaining in its own right.
None of these has a plot worth exploring, and a lot of them require the use of a guide or of magazines that I have failed to find during my play through. I didn't go searching for them, but I did make sure to keep an eye out and I never found more than one in the Doom Train side quest.
Frankly, I'd only advise you to do the GF ones and to use guides in order to complete those.
It's not worth it otherwise.