Personally, my issue with plant substrates and axolotls is iron.
(Well that and I have spent the last few weeks compiling everything you never needed to know about substrates for a number of writing projects. This research has included direct conversation with ten and counting substrate companies...) As a paranoid type hobbyists with quite a few years of experimentation with tank design, I generally leave out the fancy plant substrates anymore.
Both most commonly sold forms of this product (Known as generally laterite and flourite under numerous brand names) contain high amounts of iron. High amounts of ANY metal can be potentially detrimental to the health of amphibians in a closed environment like an aquarium.
There is quite a bit of argument both for and against these, especially laterite, with their use for plants. The argument is based on the release of plant friendly nutrients versus the insolubility of both compounds in water. In the case of their use with axolotls, it should be kept in mind that both of these substrates are designed with
warm water tropical fresh water aquaria in mind (water temp 72F or higher, bright lighting and contain nutrients targeted for plants that prefer these conditions. The effectiveness of these substrates on plant growth is arguable, based on both professional research and anecdotal evidence from many, many hobbyists. This leads us to the number one rule of vivaria design: When in doubt, leave it out. And in the case of most plant substrates and axolotls there is quite a bit of doubt.
In cold water aquaria, such as used by axolotl and goldfish hobbyists, the effects of plant substrates are generally more trouble than they are worth. The most common problem that develops is algae blooms resulting severe crashes in carbonate hardness and total dissolved oxygen. These two issues quickly cascade into a very unhealthy axolotl environment. Discussion with manufacturers of these products for the most part stated that their products were not designed for use in axolotl water parameters.
As Frank and Dr. Rayson stated previously, the true issue here is the risk of impaction related injury and death. I would venture to guess that improper choice and application of substrate is second only to water temperature issues as leading causes of death and illness in aquatic caudates. I should note here that there are plant substrates that pose little or no impaction risk. I will also state, that you can use impaction risk type substrates IF they are properly layered to keep them buried. This issue that arises in this use is as stated before: The risk of anaerobic action increases dramatically. Anaerobic bacterial action has a much more serious impact on caudates than it does on fish for the simple reasoning that caudates tend to spend much time on and in the substrate. Couple this with the gas permeable skin they have by design and you open up a whole new can of worms!
Strictly based on the factor of safety to the animals involved, I would suggest a topsoil based planted tank over a laterite or flourite based tank. Or use of the product Frank has utilized in his very successful, veterinarian approved set up.
More on this can be found here:
http://www.caudata.org/forum/showthread.php?t=62407&highlight=aquascaping and here:
http://www.caudata.org/forum/showthread.php?t=62491&highlight=planted
Quite a bit of excellent data can also be found here:
http://www.plantedtank.net/
There are also a plethora of books, both good and bad, on the subject.