You are correct in that it's more likely their system just can't scale.
Most folks I know who use RH just use it as a fun little side gamble/game. Small-ish amounts of money, something they could afford to lose, with the rest in more stable funds or investments.
On the other hand, I don't really care. I'll be fine. Most of the folks I know who use RH are just joking about it being down. It's annoying, but no one's broken up about it. A lot of the folks casting their anger at RH seem to be folks who already dislike it and probably don't use it?
Professional or large-scaled investors with large sums invested probably aren't using RH for a lot of reasons even before you get to the downtime issues.
It probably works fine, but I am never as confident on a touch screen. I even prefer to order pizza on a desktop.
I wonder if people will leave the platform en masse to alternatives as most brokers have no fees on trades these days.