The tube magazine is only an issue if you need to reload fast, right after shooting your 10th round (that's for my .357mag, a mare's legs has less capacity of course). In which case, the damned bear earned his lunch anyways, he can have me. *grins*
Otherwise, it's no worse than reloading a muzzle-loader. Not the most ergonomic, for sure, but you get used to it pretty fast. One thing to be careful about is that if you just let her 'rip with 10 rounds, that barrel can be pretty hot. Then it's a lot easier indeed to load from the side without burning yourself, lol! The reason the front loading tube went out of fashion is as much that hot barrel as the fact that it's a lot easier to reload a side gate from the prone position or sitting on a saddle, I think: the front loading tube does not make a good military weapon because of these characteristics.
I'd be curious to compare the loading times with a side gate owner, though: once the tube is opened, you can pretty easily drop a handful of cartridges down the tube by gravity, no need to push them in one by one. Overall, I suspect there's not a lot of difference in how long it takes to reload.
And those Henry's sure have very smooth lever action and a surprisingly nice trigger.