The compressed/deformed area of the crimp connector differs quite a bit on various crimping tools.
Basically, there are two crimp styles on high quality crimpers like the AMP crimpers, mentioned earlier. One being the diamond shaped opening for insulated connectors/terminals, where both sides of the crimper "die" area are the same shape.
The other style is the W type (primarily for uninsulated connetors/terminals), and half of the die area is about half-round, while the other half is shaped similar to a 3. When the two sides are forced together, the C&3 together form a sort-of W in the cross section of the crimp zone.
A third common type is a simple straight bar on one side, and about half-round on the other side. When C&- are forced together, it sort-of forms a U shape in the crimp zone. This type of crimp is primarily intended for uninsulated connectors/terminals.
This U-shaped crimp is the one that I sometimes use for insulated connectors. The crimp zone gets fully closed around the stranded wire, and a good compression takes place when the amount of wire placed in the connector, and the applied hand pressure are a good balance. The insulator becomes compromised when the bar die displaces the plastic when the crimp is completed. The plastic insulator displaces on the bar side because it can, since the plastic isn't constrained like it is on the other side (also because the surface area is less on the bar side, so the pressure forces the bar into the plastic).
The U-shaped crimp, when made with crimpers that are made from stamped, flat steel, is not as secure as the same crimp made with forged steel crimpers, because the thickness/width of the tool (also the die halves) is different.
The stamped, flat steel crimping tools are generally thick enough to make good diamond crimps, but not very good U-shaped crimps, IMO. Some stamped, flat steel tools use a V notch for the area of the uninsulated connector crimp.
The diamond and W die sets in the AMP hand crimpers are wider than stamped, flat steel crimpers, and consistently form very high reliability crimps (every time), but they are limited to 2 wire sizes per crimping tool, and cost over $400 new. These are the reasons the AMP crimping tools are basically considered specialty tools. When they can be bought for under $30, used, they become an excellent value, as long as they're in good condition.
One crimper feature that I didn't see mentioned is comfortable grips. This can be a fairly important factor when many crimps are needed to be made in one session.
The forged steel crimpers generally have wide, comfortable grips, and the AMP models (others, too) have comfortable, almost tubular handles formed from flat steel. Compound leverage is used in many of the high quality crimpers so operator fatigue is essentially nonexistent.