Known problems 

The token does not cover the payload

The token in _ss is the same for every link and appears on every search page, so a client that reads one page can build _ss values for any parameter combination. Solr Shield stops crawlers that follow or mutate tx_solr links, not a bot written against it (#6).

Bot detection excludes visitors without JavaScript

With solrShield.botDetection.enabled, a search without the behavioural payload is rejected. That includes visitors who block JavaScript.

Custom range facet scripts are not patched

Only EXT:solr's own facet_daterange_controller.js and facet_numericrange_controller.js are swapped for the patched copies (see Range facets). Range filters in your own, modified or bundled copies break with URI obfuscation until you port the changes (see Copied or bundled EXT:solr scripts).

Bot detection payloads can be replayed

The payload is generated in the browser and is not signed. A bot that records one from a real browser can send it again for up to ten minutes, or fake one altogether. Bot detection raises the cost of scripted crawling; it does not prove a human is present.