I have built two programmers. The first one for memories, about 15 years ago. The second for the Intel, Philips and Atmel 8051 derivatives, 2 years ago.

Some things I have learnt:
1) The "universal" alternative implies a very complex HW and SW. A lot of transitors, a lot of routines.
2) The "limited" option is affordable. The programmer fits in a single 160x100 mm board. Minor changes in the device under programming (DUP) pinout can be managed by SW and a small glue logic. Changes in the Vpp pin number are more complex to solve with electronics, so I decided to use jumpers. Changes in Vpp level can be managed with jumpers (for 2 or 3 levels) or with a power DAC for more flexibility.
3) I have used an adapter board to program the AT89C2051, not available in my first design. This approach is cheap and works but is not very comfortable.
4) Manufacturers frequently change the algorithms and circuits for their MCUs. As an example, I have experienced problems with the AT89C55WD recently.

A conclusion:
Building a personal programmer will give you a lot of fun. In adition you will save a lot of money. However, you will be tied by the limitations of your own design forever. Writing new routines to implement new algorithms and, what is worse, modifing your HW to meet the new requirements.
The commercial programmers use a universal HW and have a group of people working for you, writing new routines for new devices. This SW updating use to be free, at least for some years. In adition, the universal programmers expand the applications to memories, PALs, PLDs, and even chip test and recognition.

I suggest:
Build your own programmer for fun and for hobbyst applications.
Purchase a commercial programmer for commercial applications.

Regards,
Alfredo del Rio.