I have had a cordless Dremel for about 20 years that has a NIMH battery. One battery has lasted the whole time and holds a charge for many months. I have never had it go dead in storage. It is nice and light weight but nowhere near the power of a corded Dremel. It you load it too much, it stops due, I suppose, to current draw limiting circuitry. Wait a few seconds and it will run again.
For heavy work, I start with the corded Dremel tool every time.
I would stick with Dremel.
Only problem I have ever had with the corded Dremel was the clear plastic flexible drive shaft dried up and split (after close to 30 years of use) during a heavy grinding session. I found replacement drive shafts on Amazon for fairly cheap.