|Christian County, Kentucky Will Book Q, p. 172-173|
1. Not all records are digitally indexed. I was having difficulty finding anything so I tested it on a will I already had - Leonard Wood in Christian County, Kentucky 1857 (will book Q, p. 173). I couldn't find it with a name search, but did find it by looking manually. It's not a case of it being transcribed wrong, it looks like it's just not indexed at all (the button to bring up the digital index is grayed out on both the will page and the handwritten index pages). I can't even attach to my tree - the only save options are save as web link or save to my computer. Lots of people are going to miss a lot of things when this appears to only be half-indexed.
2. Many of the records were already available on FamilySearch.org. Granted, some of the records were not on FamilySearch, and the benefit Ancestry.com has over FamilySearch is that many of the records are indexed. But as addressed above, not all of them are.
3. Not all collections are complete. For example, the Philadelphia Wills begin in 1838 so anything before that is still not available online (FamilySearch.org doesn't have any Philadelphia probates). They do have earlier years for Administration records though. The date range for each county is difficult to see upfront because the collection is for the whole state, which is then broken down into counties. The specific date range for each county varies and isn't mentioned so you have to look through them to figure it out. Also, not all collections include all counties for that state. For example, the Pennsylvania Probates says it has records for about 90% of PA counties.
4. Not all collections have a manual index. For some, the odd missing digital index isn't a big deal, as many of the collections have a handwritten index in the images which you can manually browse. Unfortunately, not all of them do. The Philadelphia Wills, for example, do not.