NiMH batteries are generally very stable and safe. That having been said, NiMH batteries can smoke/burn, although not usually with the same fireworks of a LiPo battery. One of my flying buddies had a charger short out while charging a NiMH pack, and it went up in flames. It never was determined if the bad pack blew the charger, or if the bad charger blew the pack.
I had problems with a particular brand of NiMH packs (Hi Energy) not peak detecting on my Hobbico Quick Field Charger II. I'd set up my TX and RX batteries to charge while I was helping other pilots with their planes, or while instructing students on a club trainer. After a while, somebody would say, "Hey Ed, your airplane is smoking!" I melted down the same RDS8000 transmitter twice before I learned my lesson; always remove your transmitter battery from your radio when quick charging. I was fortunate to not lose an airplane when the receiver battery went up as well.
I still have the same charger and I use it all the time; it peak detects fine with other brands of NiMH packs. I guess the point is that no battery technology is completely safe or fool-proof. It's a bad idea to leave a battery absorbing energy and leaving it unattended, no matter which battery chemistry is being used.