OAuth2 allows third-party clients to use different types of grants in order to request access tokens. The specification defines a number of grant types to get some specific flows supported, but also allows for extensions - one can use whatever custom grant is required in a particular scenario.
SAML2 Bearer Assertion Profiles and JWT Bearer Token Profiles standardize two such extension grants, SAML2 Bearer Assertions and JWT Bearer Tokens grants, which can be used like any other OAuth 2.0 grants in order to request new access tokens. In addition, these documents specify how SAML2 Bearer Assertions and JWT Bearer Tokens can be used as client authentication credentials, irrespectively of the actual token grant type.
The use of SAML2 Bearer Assertions in particular is likely to get more immediate traction given that SAML2 Identity Providers are deployed in many productions, though no doubt JWT is going to be become much more visible going forward.
I think it is fare to say that it is the introduction of SAML2 into an OAuth 2.0 'space' that led some experts to start talking about the increased complexity of OAuth 2.0 compared to OAuth 1.0. One can definitely agree that working with SAML is more complex than with simple tokens.That said it is not a default token type after all - it is really about getting the Identity Providers which are already deployed be 'involved' which is always important - if that is not something that is needed for your case then no SAML2 assertions have to be used at all.
You may want to check SalesForce and PingIdentity documentation for more information on how to get SAML2 assertions utilized with OAuth2 servers.
Apache CXF OAuth2 module introduces an initial support for using SAML2 Bearer Assertions as grants and authenticators. Please check the documentation, see how easy it is to get the client code to use SAML2 assertions, start experimenting with the code and plan moving your OAuth2 project to the next level :-)