Embarking and debarking from an aircraft is relatively straightforward. You push a short set of steps up to the door and let the passengers go.
It is much more difficult with a tail-sitting rocket. Even more difficult if it occasionally lands on wilderness planets with no spaceports, launch towers, or other amenities. Compounding the problem is that if your rocket uses a fission reactor, you have to prevent the crew from receiving nasty doses of radiation while entering or leaving the ship.
If your rocket only takes off and lands from spaceports, this necessitates that the launch pads be equipped with launch towers that have elevators. The elevators can carry the crew and cargo to the loading hatches. You'd better include a way to move the tower away from the rocket before launch, especially if this is a nuclear rocket. The exhaust plume will violently terminate the tower's warranty. The rocket will be limited to landing at spaceports equipped with towers tall enough to reach the loading hatches.
Things get more difficult if you are landing on a wilderness planet. The obvious solution is a species of ladder set into the side of the rocket. The one in the movie Destination Moon had individual rungs that could retract when you needed the hull to be aerodynamically smooth. That must be a maintenance nightmare.
However, you are now limiting your landing party to only able-bodied crew people. The ladder on the Destination Moon ship "Luna" is about 23 meters (75 feet). When was the last time you climbed a ladder that was eight stories tall? If you break an arm or leg, you are stranded. And there is no way to transport any large amount of cargo.
In most rocket designs, the bottom edge of the hull is some distance above the ground due to the tail fins. This is generally to prevent the exhaust plume from splashing back onto the hull during landing. However, if your ladder is attached to the hull, the crew will have a problem with the gap.
In many SF images, they use an extenable attachment to bridge the gap, sometimes a rope ladder. In this scene from Destination Moon the crew carries down the lightweight extension. At the bottom of the hull they jump down to the ground, safe in influence of the weak Lunar gravity. Then they hook the extension onto the end of the hull.
The traditional way to deal with the cargo and wounded crew member problem is with a crane.
One of the draw-backs to a belly lander is that in the spacecraft in general, and in the habitat module in particular, the direction of "down" changes. Under thrust the direction of "down" is in the direction the exhaust is traveling, along the thrust axis. When landing on the ship's belly and while sitting on the ground impersonating an aircraft, "down" is at ninety degrees to the thrust axis.
This is Krafft Ehricke concept of how to drastically lower the delta V cost of delivering payloads to the Lunar surface. It seems crazy, but it just might be crazy enough to work.
As a slide-note, the Slide Lander concept makes a cameo appearance in Stephen Baxter's novel MOONSEED.
And if your ship is one of those specialized for water landing, embarking/debarking/load-unload cargo is a nightmare. A tail-lander floating at sea is going to have a large percentage of the body submerged underwater. It will not be able to get anywhere close to a shore-based wharf, not with its outrageous draft (draught). It will have to unload onto a free-floating wharf or a cargo naval vessel far away from the shore.
In this case it would make more sense to use some kind of belly-landing sea-worthy shuttle. That way you'll have a ship which has a small enough draft so it can float up to a wharf, quay, jetty, dock, or even a reserved berth; without ripping out the shuttle's belly on the jagged rocks on the harbor sea floor. The main ship can be a functional tail-sitter up in orbit.
Things become vastly more complicated if the engine is radioactive. Due to mass considerations, the reactor is usually unshielded, except for a "shadow shield" which only protects the habitat module. When the crew start down the hull ladder, they will soon be exposed to the glow of deadly radiation. It might be worth the mass penalty to add some side shielding to protect the ladder. Or parts of the shadow shield could be rearranged.
An interesting solution was in the old "B" movie Battle in Outer Space. Consider, if a girder is thirty meters long when it is vertical, it is still thirty meters long if horizontal. Say that thirty meters is a safe distance from the atomic drive in the ship's tail. Attach a thirty meter girder by a hinge on the atomic drive, and put an elevator cage on the other end. When the girder is vertical, the landing party enters the cage, a safe thirty meters from the drive. The girder then pivots on the hinge until the cage is on the ground. All this time the landing party is still at a safe distance. They then exit the cage onto the planet's surface. If the ship is streamlined, the girder and cage will be recessed into the hull, with the outer part of the girder covered by the ship's skin (you can see the recess hole in the middle picture above).
There was a similar arrangement in The Jupiter Theft by Donald Moffitt.
To the right is an 1961 design proposal by Douglas Missile and Space Systems for an atomic powered lunar lander, the radioactive propulsion unit is in the nose of the ship. This allows astronauts to exit the landed ship without going near the atomic pile.
The engineer known as JanJaap has made a design trying to make the Battle in Outer Space solution actually work. The movie solution would require most of the interior of the ship to be filled with elevator machinery. JanJaap's solution uses cables instead, to create a collapsible tram way.
Deltapax had an alternate idea, to top-suspend the tram cable.
Promus suggested that the cable can be replaced by a sort of ratchet in the support.