The main problem is that Ryan is said to have promised to follow the “Hastert rule,” named for former speaker Dennis Hastert (R-Ill.), requiring that legislation have the support of a majority of the GOP caucus before it is brought to the House floor.
Boehner had to break the Hastert rule whenever ultra-conservatives threatened to bring about disaster — a potentially catastrophic default because the debt limit needed to be raised, for example. In those instances, Boehner got the legislation passed with a cobbled-together majority comprising Democrats and moderate Republicans.
To keep his job, Boehner generally kept to the Hastert rule on other, less critical legislation. This is what made the Congresses he led so spectacularly unproductive…
If Ryan gets the job, he will likely enjoy a honeymoon period. But the fundamental problem — no functional GOP majority — will remain. Ryan believes government should be small. Much of his caucus believes it should be thwarted.