I say no to this idea. Big time.
There is a skill in managing support at various levels. As a player requesting it. As a player sending it. As a leader managing it. Just one small part of that skill, is not sending the wrong myth units!
I have occassionally experienced this issue, but rarely. This is because a good player/alliance will use a system that lets everyone know what god is worshiped in a city. Therefore only a foolish mistake will prevent support from landing - thats your bad.
I believe the current system works fine. It would be horrendous if every type of myth unit could be sent to a single city - covering all the defensive types, defenders already have the advantage, they dont need anymore.
Some other points I want to address - the idea that myth units are useless, or that no one uses them so they need reviving. WRONG. I had just under 200 cities on my largest account. Within that I had around 20 myth unit nukes of various types (well 3 types mainly), distributed evenly throughout my empire. I and my alliance used them often, and to devistating (not to mention fast) effect - both for attack and defence.
I also do not consider them a waste of favour. I could click to build the max myth units in a city, send it resources for next time, log back on in 2-3 hours, and repeat the process - my favour was back at max. When you manage things correctly, myth units can be produced at a fast rate.
I suppose I would admit that as a smaller player, myth units make less sense. Not everyone is a small player though, or stays a small player ;-)
Ev
Im going to have a cup of coffee immediately after I click save. Im also wearing blue socks today. So there.