Let us skip the technical jargon such as Crystalline Phase, Austenite or Ferrite. Hardening a piece of metal is just to heat it up to a certain temperature and cool it down again. Cool it down faster, it gets harder. Cool it down slower, it gets softer. Apart from the problems that the hardening process itself can be handled wrong and cause metal too brittle or crack or uneven in hardness, there are 2 major complications it may cause. 

Too Hard for Tools to Cut

Many elements contribute to how easy a piece of metal can be cut. Rigidity of the cutting tool, material type, cooling agent, tool speed, tool shape, etc. are all in the contribution list. But material hardness is certainly one of the most important ones. A normal HSS tool (high speed steel) rates HRC 60~65 (Rockwell hardness). In most cases, it handles metals with HRC 20~40 without too much problem. However, when the processed part is harder than HRC 40, the cutting tool life time become noticeably shorter, when parts is harder than HRC 45, cutting tool life time gets really short. There are tools specifically made for hard metals, for example, PVD, CBN and Ceramic tools. Some of them can handle hard steel up to 60 HRC and still keep good processing speed. But those tools are generally more expensive, some of them can not take hard impact. In practice, more often, grinding process is applied for metal parts over 45hrc, but most grinding process can handle only relatively simple shape like cylinder or flat surface and they are more expensive than CNC machining. And in some other cases, EMD or Wire Cutting or Laser Cutting are applied for hardened metal parts. 

Metal Deformation

When you heat up something, it bent, and hence goes the precision. A 10mm diameter medium carbon steel shaft with length of 100mm and HRC 40, if we only lathing, hardening and grinding it, the chance is we get a bent shaft. The tricks here is to control the hardening process to limited the deformation to a minimum level and then straighten the shaft (normally a hydraulic process), sometime aging and several time straightening are needed, before grinding.

Suggestions for Hardening Metals

In conclusion, when metal are hardened, the extra hardness and deformation caused by the heat up process requires more complicated treatment for the precision parts. Depending on the circumstances, extra process (EDM, straightening, wire cutting, grinding etc) may be needed and controlling the parameters of the heat is important for acquiring better precision.