I always steer newbies towards MF assets. Primarily because everyone "gets it" and generally less expensive to operate, barring an unseen massive capital expense. There is great opportunity to substantially increase net worth in commercial assets as the tenant credit strength and rent substantially changes the value of the building where as in a MF building, you could rent a room to Bill Gates and it wouldn't be worth any more than if you had rented the unit to a minimum wage employee.
NNN deals can come in basically a NNN structure or an Absolute Net structure. Absolute net deals are what you call mailbox money. You own the building, but Tenant has all the responsibilities, essentially. NNN deals, contrary to some of the comments, DO require that you manage them, or that someone does anyways, you can simply pass your management costs on to the tenants.
Rental rates are generally SET for 10 to 20 years but not FIXED. Meaning you have planned increases, depending on the Tenant - Pharmacies have substantially smaller increases than Fast Food or Medical.
All that being said, a NNN deal is a Lease structure and only as good as the Lease that is written for it. There are Tenants and LL's that have conforming Leases, but other than that, each deal is generally negotiated individually and you might have 3 slightly different leases in the same property so before jumping into a NNN deal because someone said it's NNN, you MUST review he Lease in great detail.
Some things to look for:
1. Term Remaining
2. Current Base Rent and scheduled increases (3% annually, 5% every 5 years, 7.5%, 10%? makes a huge difference)
3. Who is responsible for HVAC Maintenance/Replacement
4. Who is responsible for structural (typically LL on MTNL)
5. Does any tenant have an out clause after some period ( a lot of times it's 5 years with a decent penalty)
6. What is your annual cap on Controllable NNN Expenses (3%, 4%, 5%? makes a huge difference, especially when you're taking a property over).
7. How much control does LL have over Assignment/Assumption of Lease?
The list could literally go on and on. I think the moral of the story is to not jump into something without knowing what you're looking at which is true across all Real Estate.