There are a number of reasons why it might be better to choose a 
commercial, or another free, Prolog system:
- SWI-Prolog comes with no warranties
 Although the developers or the community often provide a work-around or 
a fix for a bug, there is no place you can go to for guaranteed support. 
However, the full source archive is available and can be used to compile 
and debug SWI-Prolog using free tools on all major platforms. Users 
requiring more support should ensure access to knowledgeable developers.
 
- Performance is your first concern
 Various free and commercial systems have better performance. But,‘standard’Prolog 
benchmarks disregard many factors that are often critical to the 
performance of large applications. SWI-Prolog is not good at fast 
calling of simple predicates, but it is fast with dynamic code, 
meta-calling and predicates that contain large numbers of clauses or 
require more advanced clauses indexing. Many of SWI-Prolog's built-in 
predicates are written in C and have excellent performance.
On the other hand, SWI-Prolog offers some facilities that are widely 
appreciated by users:
- Comprehensive support of Prolog extensions
 Many modern Prolog implementations extend the standard SLD resolution 
mechanism with which Prolog started and that is described in the ISO 
standard. SWI-Prolog offers most popular extensions.Attributed variables provide Constraint Logic 
Programming and delayed execution based on instantiation (coroutining). Tabling 
or SGL resolution provides characteristics normally associated 
with bottom up evaluation: better termination, better 
predictable performance by avoiding recomputation and Well Founded 
Semantics for negation.
Delimited continuations can be used to implement high level new 
control structures and Engines can be used to control multiple 
Prolog goals, achieving different control structures such as massive 
numbers of cooperating agents.
 
 
- Nice environment
 SWI-Prolog provides a good command line environment, including‘Do 
What I Mean’, autocompletion, history and a tracer that operates 
on single key strokes. The system automatically recompiles modified 
parts of the source code using the make/0 
command. The system can be instructed to open an arbitrary editor on the 
right file and line based on its source database. It ships with various 
graphical tools and can be combined with the SWI-Prolog editor, PDT 
(Eclipse plugin for Prolog), VScode or GNU-Emacs.
 
- Fast compiler
 Even very large applications can be loaded in seconds on most machines. 
If this is not enough, there is the Quick Load Format. See qcompile/1 
and qsave_program/2.
 
- Transparent compiled code
 SWI-Prolog compiled code can be treated just as interpreted code: you 
can list it, trace it, etc. This implies you do not have to decide 
beforehand whether a module should be loaded for debugging or not, and 
the performance of debugged code is close to that of normal operation.
 
- Source level debugger
 The source level debugger provides a good overview of your current 
location in the search tree, variable bindings, your source code and 
open choice points. Choice point inspection provides meaningful insight 
to both novices and experienced users. Avoiding unintended choice points 
often provides a huge increase in performance and a huge saving in 
memory usage.
 
- Profiling
 SWI-Prolog offers an execution profiler with either textual output or 
graphical output. Finding and improving hotspots in a Prolog program may 
result in huge speedups.
 
- Flexibility
 SWI-Prolog can easily be integrated with C, supporting non-determinism 
in Prolog calling C as well as C calling Prolog (see section 
12). It can also be embedded in external programs (see
section 12.5). System 
predicates can be redefined locally to provide compatibility with other 
Prolog systems.
 
- Threads
 Robust support for multiple threads may improve performance and is a key 
enabling factor for deploying Prolog in server applications. Threads 
also facilitates debugging and maintenance of long running processes and 
embedded Prolog engines. The native IDE tools run in a separate thread 
Thelibrary(prolog_server)library provides telnet 
access and the packlibsshprovides SSH login. With some 
restrictions regarding the compatibility of old and new code, code can 
be replaced while it is being executed in another thread. This allows 
for injecting
debug/3 
statements as well as fixing bugs without downtime.
 
- Interfaces
 SWI-Prolog ships with many extension packages that provide robust 
interfaces to processes, encryption, TCP/IP, TIPC, ODBC, SGML/XML/HTML, 
RDF, JSON, YAML, HTTP, graphics and much more.