I guess there is a tradeoff to be made, between timely response and massive oscillations and/or false positives.
I believe it aims at adjusting to reasonably slow changes of bandwidth, but in all honesty I never had to use it myself.
I guess the problem is that it needs to estimate the bandwidth from the arrival times of the incoming packets, so it will be partly driven by the characteristics of the incoming traffic. I believe the IQrouter guys implement something where they run RTT tests concurrently with speedtests, which allows them a better estimate of the achievable bottleneck rate, which for slowly changing bandwidth seems a better approach than relying purely on the "accidental" traffic patterns, but again, never used that either.
I believe this is on purpose not exposed, as according to codel theory (see https://tools.ietf.org/html/rfc8289#page-14 for details) target does not need to be exposed. That said, cake actually manipulates target for some of its priority tiers, which IMHO invalidates the rationale for not exposing it somewhat. That said, current cake does not allow to configure the different priority tins' interval independently, so not exposing target seems to be in line with the rest.