It was a huge deal and the plane was grounded for three month due to those battery fires.
Originally the engineers did not account for battery fires and needed to refit a compartment to seal the batteries in case a fire occurs.
Luckily it was either on approach, or they could emergency land the plane when the problem occured.
A burning lithium battery in a flying plane is one of the worst imaginable scenarios you can encounter during a flight and is a fucking big deal.
Edited to add : https://en.wikipedia.org/wiki/Boeing_787_Dreamliner_battery_...
0.5% of all 737 MAXes which exist have killed everyone on board. No other mass market production airliner in recent history has that sort of record, AFAIK.
The Concorde is the only thing that's worse. The A310 is closer than anything else at 1.35 fatal crashes per million miles vs the 737 MAX's 3.08- and that one was also designed to share a common type rating with an existing design.
The 737 MAX only looks bad because it's track record so far is that it's a death trap.
However, the problem with the max seems to be that instead of designing for failure - assuming that critical systems might fail and the plane should recover gracefully from the failure of most internal systems - they seem to have designed for success, and made the assumption that no part of the plane will ever fail. And that's obviously not realistic, and we're seeing the consequences of it here.