Main issue with original AVS - it eats too much memory.
I can easily push memory usage beyond 2 GB on 1080p video and it's just one click away from 32-bit process crashing.
Another issue - memory consumption is almost not adjustable in original AVS MT. In both "2.5.8 SVP's edition" and AVS+ memory consumption linearly depends on number of thread in use, BUT original AVS always eats its 2+ GB.
For example:
15 threads (default value for 8-thread CPUs): AVS 2.6 - 2300 MB, AVS 2.5 "SVP" - 1845 MB, AVS+ - 2000 MB.
10 threads: AVS 2.6 - 2000 MB, AVS 2.5 "SVP" - 1310 MB, AVS+ - 1580 MB.
AVS 2.6 will always crash a player unless it's not "4GB patched".
Videos larger than FullHD will not work with AVS 2.6.