I'd say a Fairphone is better supported than a Nexus (no experience with Oneplus). For example, the Nexus 5 is no longer supported, even by lineage (https://wiki.lineageos.org/devices/hammerhead/), although they do say:

> A build guide is available for developers that would like to make private builds, or even restart official support.

Edit: I should add that although I've already ordered my Fairphone 3 I've not had a chance to actually test it out yet. If anyone is interested I can post a 25 EUR discount referral link?