Are adapters unsafe? Plug G to plug F (UK to Sweden)
I'm thinking of buying an instant pot


The good news is that the specifications for that instant pot say it comes with both UK and Schuko power cables. So you shouldn't need to adapt anything.


  Are adapters unsafe?


There are a few issues with adaptors.


There are a lot of really shoddy adapters out there. 
Many adaptors do not provide an earth connection.
Many adapters try to generalise supporting multiple plug and/or socket types, but at the expense of a looser fit and/or a smaller contact area.
Many adapters have a lower current rating than the sockets they adapt between.
The extra weight and leverage puts more stress on the socket.


If it's a well-made adaptor that has correctly sized (4.8mm not 4mm) pins, the correct Schuko retention features, an intact earth path, a sufficient current rating and a socket designed specifically to take a BS1363 plug then it's probably safe enough but still for long-term use I would always favour changing the plug or cord over using an adapter. 

Also be aware that Schuko plugs are unpolarised, so live and neutral may end up swapped. For modern equipment built to IEC standards this shouldn't be an issue but for vintage equipment or home-built stuff it may be.