It's 3 different issues:
1) NBOMe's are new and therefore subject to the same concerns as ecstasy in the 90's. We don't really know much about safe dosages, long term effects, etc.
2) There have been cases of deaths and hospitalisations with 25i. It's assumed that the other NBOMe's aren't necessarily safer, just less popular. Some of the cases are due to massive overdoses, most are unknown doses (the assumption being that they were probably overdoses) and one report seems to involve a hospitalisation at a normal dose.
3) Certain people who have a vested interest in the LSD trade or are otherwise biased for one reason or another attempt to demonise what they call 'research chemicals' which they see as a threat to future LSD sales.
There are very real safety issues involving NBOMe's though. Whether these will prove to outweigh the positives (availability, selectivity, subjective effects, etc) only time will tell. For now you can minimise harm in the following ways:
1) Always be aware of your dose. Don't just take random blotters.
2) Start with a reasonable dose and work your way up once you are comfortable with the compound. Don't trust vendors to know about safe doses, there is a common myth going around that 25b is half as potent as it actually is. Starting doses vary with the compound. The following is a good rule of thumb in my opinion (some people will say these starting doses are a bit low but there's always time to move up as you gain experience with NBOMe's):
25b - 500µg
25n - 550µg
25c - 350µg
25d - 950µg
25i - 600µg
Do you know the dose of the 25i-NBOMe you have?
4) Avoid taking more than 3 times the above doses as you start to enter concern territory.
5) Don't use NBOMe's more often than once every three weeks or so as you will develop tolerance which will make dosing less predictable.
6) Treat them like the powerful tools they are. Use all drugs responsibly. Understand the risks and make an informed decision. Don't push your experiences as far as you would with other more understood substances.
7) Possibly avoid 25i in favour of the others as this one is associated with medical emergencies.