Shaving one's head is a different task from shaving one's face. Most significantly, one can see all of one's face when shaving it. The same is not true for head shaving, absent some skilled use of multiple mirrors.
The skin of the scalp is different from the skin of the face; the fat layer is much thinner, and the bones of the skull may have a few bumps or pits requiring extra attention (while being unseen during the shave). The thinner skin, the greater difficulty of stretching the skin of the scalp, and the new skills required to shave just above the ears (e.g.) make head shaving initially challenging.
Methods of head shaving
Shavers who use straight razors, or double/single-edged blades, often do not use these same tools for shaving the head. Since the lack of visibility and, to a smaller extent, difficulty of skin stretching, are both factors, shaving "by feel" is perhaps more pragmatic than trying to rig a system of mirrors. One should feel the blade's contact with the shaving surface (for some users, this activity includes the sound of the razor blades cutting hair as well) and after a pass, use a wet hand to feel for rough spots.
Advantages include: strong resistance to nicks. Speed. Disadvantages include: closeness of shave; ingrown hairs; expense. Not all electric razors can be used in the shower.
Cartridge razors are a popular choice for head shaving because they provide a good compromise between shave closeness and nick resistance. They tend to clog because of hair density and possibly length (if more than a few days lapse between head shaves), but offer generally good performance with a low risk of cuts or nicks.
Standard cartridge razor
Many wet shavers have an old cartridge razor or two around and can use their remaining cartridges for head shaving. The hair of the head is generally softer than the hair of the beard, lengthening the life of the cartridge.
The HeadBlade (and similar devices such as the Dovo Head Shaver) can provide users with a more ergonomic hand position when shaving the head. The HeadBlade uses proprietary cartridges and offers shaving lubricants and skin treatments as well.
Double-edged or single-edged blade
Avoiding redundancy of shaving gear is a benefit, plus the wetshaving skills and "touch" are maintained when shaving the head. Having to proceed by "feel" for much of the shave is a daunting task. Using a mild razor is recommended, at least at first. Similarly, heavier razors might not be the best choice for head shaving. Mapping one's hair growth is of some benefit. Disadvantages include: speed, risk of nicks.
Some DE razors and SE razors have blade guards that extend slightly outside of the corners of the DE or SE blade, reducing the chance of nicks.