Both ignition systems on sidelocks and inlines are reliable.
The main problem is not the reliability of the action, but rather the source of ignition. Thats where inlines gain an advantage. Many models can fire a 209shotshell primer, musket cap, or standard #11 primer with a nipple/adapter change.
If you want the advantages of a hotter ignition system but prefer a sidelock, TC makes the Firestorm. This rifle can take advantage of hotter ignition systems and can utilize the Pyrodex pellets reliably.