While managing gems and application dependencies you must take into account that all gems installed in a hosting are accessible to all applications hosted on it.
This means that installing two very different Ruby applications in the same hosting account might not be a goog idea. Each application will have its own dependencies and properly managing those its usually a hard task.
If all applications use a correctly defined Gemfile the dependency issues will usually be very small or non existent. But if there are no Gemfile or if they are not correctly defined you may create a "gem hell".
So the easier way to avoid a "Gem hell" is to host a single Ruby app per hosting account. You can form example use a reseller plan and host each application on its own hosting account.