  Let $k,n,m \in \mathbb{Z}^+$ integers such that $k\leq n \leq m$, let
$\mathrm{G}_{n,k}\in \mathbb{F}_{q^m}^n$ be a Delsarte-Gabidulin code.
  Wachter-Zeh proven that codes belonging to this family cannot be efficiently
list decoded for any radius $\tau$, providing $\tau$ is large enough. This
achievement essentially relies on proving a lower bound for the list size of
some specific words in $\mathbb{F}_{q^m}^n \setminus \mathrm{G}_{n,k}$.
  In 2016, Raviv and Wachter-Zeh improved this bound in a special case, i.e.
when $n\mid m$. As a consequence, they were able to detect infinite families of
Delsarte-Gabidulin codes that cannot be efficiently list decoded at all.
  In this article we determine similar lower bounds for Maximum Rank Distance
codes belonging to a wider class of examples, containing Generalized Gabidulin
codes, Generalized Twisted Gabidulin codes, and examples recently described by
the first author and Yue Zhou. By exploiting arguments suchlike those used in
the above mentioned papers, when $n\mid m$, we also show infinite families of
generalized Gabidulin codes that cannot be list decoded efficiently at any
radius greater than or equal to $\left\lfloor \frac{d-1}2 \right\rfloor+1$,
where $d$ is its minimum distance. Nonetheless, in all other examples belonging
to above mentioned class, we detect infinite families that cannot be list
decoded efficiently at any radius greater than or equal to $\left\lfloor
\frac{d-1}2 \right\rfloor+2$, where $d$ is its minimum distance.
  Finally, relying on the properties of a set of subspace trinomials recently
presented by McGuire and Mueller, we are able to prove that any rank metric
code of $\mathbb{F}_{q^m}^n$ of order $q^{kn}$ with $n$ dividing $m$, such that
$4n-3$ is a square in $\mathbb{Z}$ and containing $\mathrm{G}_{n,2}$, is not
efficiently list decodable at some values of the radius $\tau$.
