with too much detail
Yes, there's more detail than you need, but only the ISSOM shows where you can actually go.
and fussy rules
Rules are a feature of the competition, not the map. If you use OS/google/OOmap, you're not placing restrictions on where people go. If you think thats acceptable, then using ISSOM in an event where "uncrossible" is advisory must also be fine.
The main newcomers problem with ISSOM for Runchallenge is that it doesn't have streetnames: but then nor does OS.