I think I may expand upon this just slightly to allow two Trigger Events in one Trigger to use Periodic. Due to the design of Cron, anything that doesn't work can often be done with two Crons.
For example, the 90 minute problem can be solved by having two Periodics each running every 3 hours, but 90 minutes apart. RM could do that if I allowed two Periodics.
Every other week, however, in the literal sense, is not possible. Doing First and Third week of a month is fine, but won't yield every other week exactly, due to the 5th partial week in some months. This really is a limitation from Cron's design, as it doesn't treat week as an element of a cron expression, only day of week.