SPAC's are incredibly risky. When they IPO they basically are shell companies with no operating revenue. So their valuations are essentially fake (some get better market caps because the people who create them have "track records"). Once a startup converts into them their valuation becomes much more interesting. It's a huge gamble. A SPAC that hasn't converted yet is essentially a guessing game. If the SPAC is valued at $500M pre conversion and the market doesn't think the company is worth $500M then you can lose a lot. The opposite is also true.
Note - its much more complex than this, but this is the general idea.
And I think it's pretty random which ones have lock-ups and which ones do not. Generally speaking their lockups are more liberal than traditional IPOs, correct? So it may depend on how disciplined the company's large shareholders are.
No special access is needed to trade SPACs, just need to follow the news. That being said, I think that most SPACs are pretty bad deals.