There are some drop structures along the way, the water will probably be very swift and powerful compared to what they're accustomed to. Also, if they go to Union, they'll likely get chundered in the Union Chutes. If they continue to Confluence Park, the low head dam at the Zuni power plant could be fatal if not portaged. Things may get stinky below the Englewood Wastewater Treatment Plant outfall.
The scenery and wildlife along the reach between C-470 and Union are surprisingly pleasant for urban boating. I'm not familiar with the rest of the trip from Union to Confluence but have done fieldwork by the Zuni dam.
Nothing in the world is more yielding and gentle than water. Yet it has no equal for conquering the resistant and tough. The flexible can overcome the unbending; the soft can overcome the hard. - Lao Tse