Am I crazy going for a new AI startup instead of a decent/stable corporate job?

I'm currently busy at a rather decent corporate job at a large healthcare software business. I do:

• Data warehousing
• Analytics
• Occasionally a bit of ML

I created a framework + set of distributed services that process all the data flows there. There's a team of people writing code and deploying software in that framework. Fun and interesting. Disadvantages are that:

• Management is very chaotic. A lot of people don't know what's going on or who's doing what. Some good quality people have left.
• They only use Microsoft software/APIs. It's also very hard to get permission to contribute back to open source. For example, we maintain a private fork of a certain framework, costs a lot of time only because we can't merge back or create PRs for the changes we made.
• There's a need to interface with very old software (think 20+ years old, w/ associated problems) and bugfix/change that software if there're issues. There's an ORM from the 90s there, still maintained, ported through 3 different languages.

There's a startup that made an offer. The startup is funded and is doing a lot of very interesting stuff. Things like traffic accident prediction, reducing pollution. All through predictive models/ML. They mostly sell to governments. They have their own platform. One of the few really interesting businesses in the area.

But yeah, advantages:

• Smart colleagues
• Ability to contribute to open source
• Implementing ML models
• Diverse software stack
• Good for my resume. Maybe in 4+ years, I can go the consultancy route.

My trouble is that:

• I'll mostly be doing data engineering and software development. Probably not a lot related to data science/ML, something I want to get better at. I will implement models they come up with though. They have enough data science people, even if I do have a Math & CS degree.
• Salary is kind of hmm. I'm in Europe where salaries are a bit crap. I currently earn about 52K, which is .. ok for a senior, but not amazing. They offer a little bit more than that, with a promised profit sharing scheme once I'm there for 6 months. Trouble is, end of year salary negotiations are coming up which means I'll probably earn more than that in a month. Salary growth is decent at my current job. I feel a little bit taken advantage of.
• Uncertainty.
• Probably a lot more late nights
• Leave my cushy position where I have a reputation for getting things done and getting a lot of nice challenges. I'd feel a dick to my coworkers who'd have to pick up the slack. They've always treated me relatively well. Not looking forward to quitting.

I think I'll probably take it, do something fun, maybe fail. Learn things and increase my salary to a better level later in life.

Am I freaking out over nothing? Reasons to stay or go? Thoughts?

Also, what if the current employer makes some sort of counter offer, lot higher salary, etc. Don't want to burn bridges with the other company. How do you handle things like that gracefully?