[Gondolin]

On the other hand, constructive set theory is still rather a niche part of mathematics (this may change with the recent development of HOTT). It is hard not to use the LEM (and I am not good at it, I am not an expert on this subject. Expert say it gets easier over time to get used to not use LEM).

For instance, in the intuitionist reals, from "x >= 0 and x != 0" you cannot conclude that "x != 0". To prove x>0 you would need to exhibit a natural number n such that x>=1/n (so in other words you need a constructive proof that x>0). But this is not always possible, you have models of R where there is an x != 0 which is smaller than all the 1/n.

There is however one nice feature of intuitionist theory: every geometric sentence which is provable in classical theory is actually true in intuitionist theory. So if you have a geometric sentence (https://ncatlab.org/nlab/show/geometric+theory) you can use LEM in your proofs, the proof will still be valid intuitionally!