Short answer is no...you're not missing anything by staying with your current version of Surfcam (unless you like multiple PC crashes per day).
However I'm not sure how Surfcam handled chamfering and threadmilling in your version. The last version I used was 2015 R2 but 2 of the programmers here are on 2020 now. Chamfering was still basically a contour with offsets that would run all over a sidewall if the geometry wasn't drawn correctly to avoid it. Threadmilling was fairly straight forward.
Posts are probably still the same with the exception of some rapid logic added but for a 3 axis Haas you can't find an easier post to modify to your liking.
No automatic part nesting has been added. You still need to transform/ copy.
No automatic tabbing feature has been added.
When we started doing more 5 axis and millturn work some of us transitioned to TopSolid. It wasn't an easy transition but it was worth it in the end for us. Surfcam is still faster for the easy stuff though.
I honestly wouldn't bother upgrading for 2.5 axis stuff.
Another thing to consider is if you do upgrade, you may not be able to open your older Surfcam files with the new version. I believe they changed the extension type to .scprt in 2014. There were hacks to get around that but they weren't reliable.