At the moment, clangd and ycm are very similar projects; they are very limited compared to cquery. clangd and ycm support code completion, diagnostics, fixits, and goto declaration (but not definition), whereas cquery supports references, derived types, callers, etc. Basically, if the feature requires knowledge across multiple translation units, clangd/ycm do not support it.
cquery is designed to support very large projects, so it makes very specific design decisions w.r.t. the data model, indexing pipeline, and multithreading model. I hope clangd can match the performance - but so far every project I've seen simply does not run nearly fast enough on a code-base the size of Chrome/ChromeOS.