Top panel: number of GCs as a function of the host galaxy stellar mass. Filled purple circles are the new UDGs in our sample, while filled black crosses are Hydra I new LSB galaxies. Filled red circles are Hydra I UDGs from Iodice et al. (2020), and the orange thin diamond is UDG32 (Iodice et al. 2021). High-surface-brightness and LSB galaxies from other low- and high-density environments are plotted for comparison. We show the Fornax and Virgo ETGs (Liu et al. 2019; Peng et al. 2008, respectively), the Fornax low-mass ETGs (Prole et al. 2019a), the Local Volume and Virgo low-mass ETGs (Carlsten et al. 2022), and the MATLAS-2019 UDG (Müller et al. 2021) (markers are indicated in the legend). Bottom panel: GC-specific frequency S_N versus V-band absolute magnitude for UDGs and new LSB galaxies in the Hydra I cluster (symbols as in the top panel, Iodice et al. 2020, and this work), compared to the UDGs in the Coma cluster (green triangles, Forbes et al. 2020a), and to the UDGs in the Virgo cluster (blue squares, Lim et al. 2020). Virgo UDGs with a negative specific frequency are reported at S_N = 0. In all three cases, GCs are selected within 1.5R_e and are locally corrected for background contamination. The solid line shows the mean locus of dwarf galaxies, the dashed line represents the upper 2σ bound (see also Fig. 4 in Lim et al. 2018). The average uncertainties on Coma and Virgo are shown on the left side.