"How bad" is entirely dependent on what kind of a captain the sailor had, what his ship was engaged in doing, and what the conditions of his impressment were/what his previous naval service was like. Impressment was certainly a harsh way to man the navy, and was resisted more as time went on, but the lot of the common sailor varied quite a deal depending on their command.
in that you'd have no contract end date,
This is correct -- men were not entered for a term of service, but for a "voyage" that would end when the whole ship was paid off at the end of a mission, which could last for some years of service.
often weren't properly paid,
Sailors' wages lagged behind merchants wages for most of the Napoleonic period, due to simple supply and demand -- the British attempted not to raid their merchant fleet for the Navy, even giving protections to certain professions such as whalers, but some officers would impress merchant sailors regardless as need pressed. The "not adequately paid" probably refers to the practice of men being paid in tickets/bills that could be cashed only at the Navy Office in London, and would usually simply be sold locally at a discount to speculators.
and may never have a chance at liberty again.
Indenture wasn't slavery (though it was involuntary servitude), and certainly indentured men could be wounded or killed in action, but the term of a man's enlistment would be up eventually.
I've written about sailors' lives a lot, which may be of some interest to you:
https://www.reddit.com/r/AskHistorians/comments/2oo91z/during_the_age_of_sail_how_reliable_were/
https://www.reddit.com/r/AskHistorians/comments/1kmyo6/during_the_age_of_sail_how_did_life_for_both/
https://www.reddit.com/r/AskHistorians/comments/5ujcsb/was_it_common_for_sailors_with_physical/
https://www.reddit.com/r/AskHistorians/comments/9scxhz/in_comparison_to_your_average_trained_navy/