Note that as more senders use mail service providers, comprehensive blocking of all SPF records may have considerable collateral damage.

Whether you consider this good or bad may vary: it means MSPs have a valid claim that senders exercise Good Citizen practices on all mailings, not just those circulating via the MSP.

Then there are the major players. Yahoo's been a freaking PITA for years on outbound spam (both spam and joe-job bounces), with no effective reporting mechanism. Gmail's been largely better IME, other providers vary.