there is no catch - axoloti does what it says on the tin (or rather the specs )
and its great you are enjoy it!
could you replace everything with it, errr probably not...
(for reasons you have already stated really)
this is kind of true... and something that is easy to underestimate.
many hardware synths, and vsts are hugely complex things, that have take a long time to develop by experienced developers - so for you to do the same too, would naturally take a long time.
and of course, if you don't have the skill set of these developers, then you'll need to learn too...
sure, if your plans include things that have already been developed, either in the factory or community library, then this helps - but DSP is a large area, not everything is available, and also you might feel it needs tweaking.
AND you need to know how to combine and use these things.
well axoloti is a microcontroller, so this has limitations (as per spec), its a 168Mhz arm processor, with 8gb of sdram - this is pretty powerful in itself, as this is all focused on your patch, but this is orders of magnitudes less powerful that your desktop/laptop.
for sure, you could buy multiple boards, but some 'operations' are indivisible (you cannot spread across boards) e.g. FFT
(but to give some 'context' to this... Axoloti uses the STM32F4 microcontroller this is the same chip as used by mutable instruments in both clouds and elements - two of the most popular eurorack modules !)
so this is analog, which some like the sound of... and analog modelling is really computationally expensive.
can you do something 'close enough' on axoloti?
well only you know... nothing is right or wrong, its what you like
now none of this I see as negatives , or a catch...
in fact, id say the more you understand/accept/appreciate the limitations - the easier it is to appreciate axoloti's many strengths.
its amazing to see what many have here have done with axoloti - and also the diversity of what they have done too.
also the key for me, for axoloti, is it can be what you want it to be... its not about replicating something else that exists, rather creating something of your own, with the features you think are important, rather than another synth designer.
( I also personally find it quite educational, I get a better feeling for why things are designed the way they are)
anyway, have fun, and happy patching !