I have to completely agree on Great Art being very hard to theme, and Artifacts being better to use. However, when theming Artifacts, you may have a couple of unthemed museums, because you can't really control from which age they come from. At least with Great Art, you may use Wonders to store Art until you're able to theme them, so you don't lose that much. You can also keep them there if your museums are already full, until you build another.
But I can hardly say that the other types are underpowered. Great Writing is plentiful throughout the game, both slots and Great Writers, and after Printing they give double tourism. For Great Music, there's a policy that triples their tourism, and you'll probably have enough slots late game. It's not like you need to build much after Museums, so channel that production into Broadcast Towers (granted, their cost should be lower, they're so damn expensive).
Maybe you could enable Writing theming when you research Printing, instead of double tourism, though.
As for the seemingly irrelevant Great People, I usually rush to Computers after Humanism, so I'll have a number of Campi generating GPP towards Mary Leakey. For Scientific players, well, you deny a good chunk of tourism from cultural players. It forces you to decide whether to save points for better people, or play defensively. And congratulations if you manage to get the last Great Merchants before victory. In my opinion, they come way too late to have a significant impact.