The short answer - is as dangerous as you want it to be.
There are some roving bands of thugs, living like pirates on the high plains. But remember, this is five years on. To survive the other hazards of the zone - food, water, weather, reanimates, and low enforcement, they had to be more than just some neighborhood gang. In short, real leaders, with plans that go beyond just attacking anything in sight. Many have become stable warlords, taking tolls enforced by technical (trucks with mounted weapons) rather than launching risky raids on established settlements. Along the Mexican border are entire fortress towns run by former cartels, while up in Canada, those with the guns control the fuel and heat necessary for making it through the winters, and are thus the de-facto leaders.
Reanimate numbers are really hard to gauge. They're almost a quantum phenomenon - no observer, no neighbors. Drones flying through old cities won't pick up much activity, nor will the initial scouts see all that many milling around. Yet, the longer an area is occupied, the more seem to show up. Some have speculated that they remain in a kind of stasis until potential prey comes into the area. Increasing numbers are activated until the intrusion is removed. There is still no explanation for why they act like a macro-immune system however.
Citizens are out numbered, often out gunned, unfamiliar with the terrain, and politically divided against themselves. Even presuming 90% of Earth's Human population died between 2050 and 55, that would still give humans a better than 20:1 advantage. When they leave their settlements in armed groups, its as often for their own protection as to achieve some military objective. however, there are groups of them who seek to expand their territory, or remove potential competitors before the enemy has time to organize. The ones in California are known to be quite territorial - almost no one is able to reach the west coast.
Most of the citizen ships seem to be in stable lunar parking orbits, and otherwise unmoving and un-illuminated. Its unknown what power-source they use, but interstellar travel must require a lot of energy - and thus they may simply be out of the fuel necessary to function. Others speculate that important pieces of equipment were damaged - either by prior happenings (which forced the Citizens to come here), something during the event, or perhaps even deliberate sabotage to keep them from being used in the developing citizen civil war. So long as the ships don't enter low earth orbit for bombardment purposes again, most people are content to not think about it.
Any industrial area that is likely to have a containment failure probably already has, or has reached a stable point. After a week, the heat of a shut down nuclear reactor is only .02% of what its active rating is, and continues to slowly drop from there. However, exploiting these plants is going to eventually become necessary as part of restarting the machinery of industry and civilization.
Diseases, by their nature, need hosts for transfer and incubation. Although it would be wrong to presume them extinct, after five years of no transportation and frightfully low populations, many of the truly worrisome viruses have died off with their carriers. While cholera, typhoid, measles, malaria and the like will never go away, it is unlikely to discover any of the "2020 generation Vectors" that created the absolute need for Semi-Volitional Internal Agent augmented immune system. (Nano-vac) Unless of course, people go poking around in research hospitals seeking medicine or nanotechnology.
So its Safe?
Of course not. But it is not a place of small groups vs the world. Old warehouses continue to be old warehouses, not dungeons full of creatures that must be liquidated to retrieve the treasure. Parties don't need to grind against random encounters every time they step foot out of town. But things are rarely so calm as to go without a hitch.