* "encoded" may be the wrong term (someone will correct this if I'm wrong).
Nope. That's right.
I may save about 30% space out of a 40GB BD converting to MKV
Converting to MKV using MakeMKV does not save any space over ripping to ISO (nothing substantial anyway), except if you don't rip the extras. Assuming you rip everything available on the disc, they should be roughly the same size (save the size required for the menu system, I suppose, and maybe slightly more efficient storage). Certainly not 30%.
I suspect you were ripping using a
different application that re-compresses the video to H.264 MKVs and shrinks them to save space, and remembering wrong. This can be done well, but is "lossy".
MakeMKV
cannot do that. It only rips the original files and decrypts them, and then "wraps them" into a MKV. It
does not re-encode them, and is also quite quick (for this same reason). It usually rips BluRay movies on my system in 20 minutes or so (though this depends on the film in question, and the size of the rip).
The main benefit to ripping to ISO is that it preserves the menu system. This is certainly a benefit, I suppose, if you like that sort of thing. To me, it is a "good thing" to get rid of those horrible things. But using ISO rips does also have substantial file management and ease-of-use limitations.
It
is a trade-off, but
not a quality trade off, was the point I was making.