I'm not sure what your plan is with the "userX", but Maemo is pretty much designed for single-user devices, and it's not straightforward to make any use of additional users.

As for root giving you a root shell, that's what rootsh does, in addition to permitting sudo gainroot. (su itself is not permitted at all, because all escalation to root is to go through sudo, and policy established that way.)

Taking "not so save" to be an inquiry about security, physical security is everything. Don't enable remote logins as user or root, or at least only with adequate password/crypto, and then the ability to escalate to root at will from the console is not much of an issue. (If they get hold of your device, they'll do what they like sooner or later, anyhow. Having root access available just saves them 2 minutes.)