Decryption and signing require private keys. Encryption and signature verification only require public keys.
If you happen to be in possession of a private key, there is one thing you should know: that key is private! It shouldn't leave your computer and most certainly should never be uploaded to any website!
So yes, alternative services may offer more cryptographic operations but at the highest cost of surrendering your private keys to servers you generally shouldn't trust and companies that may be under geopolitical influence.
-Keyoxide offers a simple solution to the trust issue: we don't want your keys, therefore you don't even need to trust us.
+Keyoxide offers a simple solution to the trust issue: we don't want your keys, therefore you don't even need to trust us. Everything that this service offers is possible thanks to publicly available keys.
We still need keys, of course, but only the harmless public keys. And yes, we could have built a website where one can make an account and upload public keys, in a similar fashion as alternative services.