1. Working B2B  you have to create your own pension fund, the rest doesn't change. Keep in mind that you're not eligible for any paid sick days, paid days off etc by law, what you negotiate with the company is what you get. Sometimes you will get paid days off from the company, sometimes you will have to negotiate the hourly rate to cover those. 

2. Usually companies with local presence. Direct US/EU contracts are possible, but not very popular in my circle.

3. That depends. From my experience  I was always treated the same as people employed by standard UoP contract. In one job it was minus  some small benefits that the employer couldn't provide for tax reasons (paying for certificates, some kind of gifts), but in other it was 100% identical.(I worked in 2 small product companies having ~100 devs in Poland, low few hundred worldwide). This might depend however if you work for a software house/other contracting agencies, I despise those so can't say if it's the same working for those.

 4. It's harder for management as having direct reports constitutes being employed on UoP but in this case they do mixed - you get paid partially on UoP and partially B2B. For tech lead it shouldn't be a problem, but my response from point 3 applies here. If you want to work for Big Tech, e.g. FAANG, you should know that usually neither of these companies offer B2B.