In my experience, a non-finetunable closed source API is a complete nonstarter for a large fraction of possible use cases, especially the higher value ones like law and engineering. Most of these embedding models are trained too much on colloquial use of language on the internet that has little overlap with how terms of art are used, and without the ability to fine tune it to a specific use case, they're only really useful for generic use cases and even then they can be limited.
Hey!
Since we focus on Enterprise use-cases, we made sure to include training data from domains like you mentioned above. While in very specific use-cases finetuning may be helpful, but we also do offer that as a customization service (just not available via SaaS)
Have you looked at fine tuning linear adaptors to sit on top of the embedding models? This works with any model (proprietary or open) and I think in practice this is significantly easier to implement anyways.