https://www.thevespiary.org/rhodium/Rhodium/pdf/sodium.cyanoborohydride.pdf (https://www.thevespiary.org/rhodium/Rhodium/pdf/sodium.cyanoborohydride.pdf)
https://www.thevespiary.org/rhodium/Rhodium/pdf/nabh4.carboxylic.acids.pdf (https://www.thevespiary.org/rhodium/Rhodium/pdf/nabh4.carboxylic.acids.pdf)
for example. Another limitation is that tryptamine and aldehydes under acidic conditions undergoes a so-called pictet-spengler cyclization, forming a tetrahydro-beta-carboline instead of the desired product, therefore all common reductive aminations using acid conditions won't work.