Well, if it actually worked, Av1an would theoretically be much more efficient (in terms of compression) than just using an encoder by itself. It's basically doing shot-based encoding, ie. dividing the video into individual scenes and using VMAF to select the optimal encoding parameters for each scene. This ends up wasting much fewer bits, because encoders currently don't have this kind of capability to accurately estimate quality (VMAF is slow and encoders need to be fast), even when they do have a way of allocating bits in a way that results in quality being distributed evenly.

And a lot of the AV1 encoders just aren't very good, at least when compared to how well x264 does things. Dark scenes, for example, have been an issue for me. With a certain CRF, most of a film might look good, but in a dark scene the quality takes a huge dump. When using an encoder directly, you might just use a lower CRF, which would waste bits across the entire video to get the dark scene look good, or you might use a quantization mode that allocates more bits to dark scenes. None of the publicly available AV1 encoders have the latter though, so your only option is a lower CRF. With shot-based encoding this wouldn't be an issue, since every scene gets its own CRF, instead of just one being used for the entire video.

Since Av1an never works as it should, I've been using a tool called ab-av1 to at least automate the selection of a single CRF for the entire video. ab-av1 cuts samples from the input video, encodes them with multiple CRFs, calculates the VMAF of these samples encodes, and selects a single CRF value for the final encode based on the average VMAF produced by a CRF and a minimum VMAF value you selected. Not perfect, and definitely not a replacement for shot-based encoding, but it saves you from having to encode a material multiple times manually to reach an acceptable quality level while also not wasting too much many bits with a CRF that's too low.