I was a consultant for the first 13 years or so of my career, then switched to working on one product. I agree with everything he said, but there are downsides too.
The downsides for me were:
* You rarely (almost never) get to work on multiple iterations of a product. You parachute in to get something working before launch, or fix the work of some other contractor, or (in the most fun case) to take an idea from zero to launch. But, in all my time, I never got to do the version of that, where we learned what didn't work and then made changes to improve things after launch. That's a huge part of product development, but it's simply not part of the model of consulting. After completing your contract, you'd Fulton out of that company and drop in to a different contract. If you want to stand by your work in the long term, and have the feeling of shepherding something to success, you won't get it as a consultant.
* The flip side of always learning is that you have to learn whatever stupid stack you've inherited from the client. Always different. Yes, you will always be learning, never in your comfort zone, and if you work with startups, you may even be on the cutting edge. But, that can also get real old after a few years.
* The business side can swing wildly from feast to famine. When I was an IC, I would spend weeks not doing anything because there wasn't a contract for me. Can be fun to sit around, but the novelty wears off when you realize your job may be on the cutting block 4-5 times a year, just due to what lineup of clients are coming through the sales funnel at the time.
* Related to the above, I think there is just a fundamental problem with the business model for consultancy shops that will tend to make them unstable in the long term: you have to staff up to get new contracts, then when those contracts run out, you either lose those employees (if they are not salaried) or spend a ton of money as they wait around for the next contract (if they are). You end up going for new contracts, but if those contracts don't exactly match your roster of available staff, you have to hire new people. As time goes on, you end up making bigger and bigger bets with staffing, and eventually you lose. I saw this happen so many times. The result is not that the company goes out of business, but that there are huge layoffs, with such a massive change in company culture, that it is effectively a different company afterward.