I think subject-matter experts make the best PMs, however, that's not a prereq for selecting a PM. 

(1) Estimating definitely improves with experience as a PM and as a SME but is heavily influenced by company/culture and the process being used (waterfall, agile, wishful). A SME PM is better able to challenge estimates - but isn’t responsible, by role, for producing the work - so not always in a position to challenge those estimates.  A PM in that situation could propose alternatives to improve estimates or challenge with estimates from a 3rd-party. 

(2) There will be impediments; typically a result of changed conditions or a root-cause in the project set up. All projects that fall behind do so in the beginning.  allow for ramp-up at the start. This is really a matter of getting the core, essential, measurable goals right in the beginning and applying reasonableness to the plan before starting.