MyRocks is LSM based. The LSM is different from BTREE in what you need to do multiple "physical" lookups and merge result to perform one logical lookup. There are some tricks like Bloom Filters are employed to optimize it but it is still not as efficient in memory as BTREE especially for range lookups. Here is some information on WIKIPEDIA https://en.wikipedia.org/wiki/Log-structured_merge-tree
Myrocks uses a clustered key for the primary key from what I have read. Isn't this the same as innodb? Is it not a B-Tree?
Could you elaborate on what's special about indexes - primary or secondary in Myrocks?