I've had inconsistent sizing with 5.10. Some size 9.0 US (42.5 EU) seem to fit comfortably roomy with space to run thick socks. Size 8.5 might fit quite snug and very conforming, but lose this feel as it picks up moisture, while others feel cramped the moment I put it on and have to walk around 5 minutes to stretch them.
And then there's differences in how they walk. Some have the front angled up so much that it sort of feels like I'd tip over forward before it ever contacts the toe part of the outsole. Others have it angled like how I'm used to.
5.10's not the only brand that this happens with. For clipless slippers, it's the same, with 42s sometimes being too tight, and 43 often being too roomy to have "foot dexterity". Definitely recommend trying everything, even if you're familiar with a brand.
Also note that 5.10 has different varieties with the same name, but in different colors. They not only differ in color, but also in ventilation. I personally avoid 5.10s which don't have a mesh top for the toe, being in a relative arid climate. I think I passed over the Freerider Pro models because they all have leather over the toe with holes to vent and looked prone to roll ankles. I like ones with mesh, since the sides also are mesh vented, and the hybrid mid/high-top style (high top on inside of ankle, and mid on outside), and the burlier outsole (tends to have some animal print on it).