As has already been pointed out here - read the reviews and spot the many fake 5 star reviews. Some are so obviously fake it's laughable. It's like watching the world's most wooden actor. It's a shame because, whilst I think it's the worst company I've ever worked for, I think there are some genuine positive reviews too - but they are drowned out by the countless artificial ones.
Anyway - why so bad? Where to start? Product is poor. Doesn't compete with the likes of Power BI or Tableau but costs way more. But if it was only that it'd be ok - not everybody can have the best product and there are some fringe use cases where Sisense can make sense.
But to summarise my experience - joined Sisense as a reasonably happy and healthy individual. Left requiring regular psychology sessions and treatment for anxiety and depression and whilst I don't lay all of that on my time at Sisense I'd say 90-95% is directly attributable to that.