For one thing, they're by definition tied to a shitty blockchain and you have to deal with any potential bullshit affecting said chain including fees, slow transactions, potential scams, etc. For another, they're not really an object, just a bit of json pointing likely pointing towards a flat image hosted somewhere else (that anyone can freely access via the link). Sure, they enjoyed a brief popularity and lots of media attention, but already the NFT marketplace seems to be waking up to the reality that most of these "investments" will never turn a profit. And that's the real issue I see: most of these purchases were not someone who wanted to collect something, they were made by someone who wanted to invest in an asset that would later be worth something. And so the actual value as a "collectible" has been inflated and is due for a huge crash, which is already happening.
I know some are meant to carry benefits towards the holder, like exclusive access to X service or community... that is a small minority currently though, and many of those benefits are shaping up to be just empty promises, and even then an NFT seems like a poor vehicle for systems that could have been done better without any "decentralization" or crypto integration.
NFTs are worth nothing to me, but so are celebrity signatures. That doesn't mean they are generally worthless b/c people collect all sorts of stuff.
NFTs are a bit like celebrity signatures. They are worth what someone will pay. If a celebrity verifiably digitally signs something and they only do it once, that is worth something to some people as a collectible. It is not worthless as long as someone is willing to pay something. And, like other collectibles the market can have huge swings and sometimes never recover (like all my baseball cards lol).