Problem #1 - Similar speed
One problem is that DOS versions of Laplink require the use of computers that are running at a similar speed. For example, if I connect my 6 MHz IBM 5170 up to my 486DX-66 clone (via serial or parallel Laplink cable) and start transferring files, I find that after about a minute, Laplink goes into perpetual 'timeout' (continually displays 'timeout'). If I then switch the 486DX-66 out of turbo (goes to 8 MHz), then restart Laplink at both ends, all goes well.
Others on the Vintage Computer Forums see the same problem.
Problem #2 - Version incompatibility
I can not get different versions of Laplink to communicate at all. So for example, I can not get Laplink 3 to communicate with Laplink 4, and I can not get Laplink 4 to communicate with Laplink 7.5 (for Windows 95). It is as though the authors have made changes to the transfer protocol at each version, dropping support for the earlier protocol.