Short for non fungible token for those that asked. Non meaning not, fungible meaning can't be split or replaced* and token meaning representative item/mark.
*money is said to be fungible. One £20 note is as good as any other where these are not so easily substituted.
Variety of reasons for the dislike.
1) Most offerings thus have been complete scams in as much as they are hyped beyond all reason.
Theoretically a cryptographically backed ledger of ownership could be a useful thing**, as would the same for contracts of which this is a subset of, but very little of any real substance has been issued thus far. A lot of money has been thrown around but speculation and if you want a fancy economic term then see "greater fool".
2) Some attribute the rise of crypto (currency) of which this is a related concept with the GPU market being hard to find GPUs as GPUs are no longer pixel pushers but actually massively parallel computation devices (see GPGPU if you want a term to search for). A weak argument from where I sit (NVIDIA and AMD/ATI need to make more chips and Intel need to make good ones) but a notion that was floated.
3) Power usage. Some people are filthy hippies and calculate power usage per and consequently get upset. Overblown and way bigger issues out there if you care about wasted power and finding jungle homes for the whales but it is a thing you can be concerned about.
4) Some artists found their work stolen. So far so internet but it is a thing people noted and got bent out of shape over.
**rather than having to press a DVD of the best of your youtube channel (minimum run about 1000 in most places, and having to ship it afterwards as well as sort payments) you could issue an NFT of it, sell that as valuable and those with ownership of the thing get to enjoy it and then sell it on in 10 years when you do the usual big youtube, big depression, lift your head up high and blow your brains out routine. More generally while we could go to court and debate whose emails said what and constituted an ownership contract with what terms then having it on a nice secure public ledger makes life a lot easier.
I would also note several of the NFTs made have actually been less than ideal here and derisively referred to as links, some actually were links rather than tokens of ownership of a work but that is an implementation issue.