Expandability:  There is no theoretical limit to the size and lap length of either a conventional or a digital layout.  However, in either case you need additional wiring to ensure uniform distribution of power to all parts of a circuit once the lap length rises above 30 feet or so.  You can add additional power taps to your layout with snap-on wiring sets made by the race set manufacturers, so dealing with this issue is not difficult.  Most digital tracks have another problem to consider, however.  As lap length increases you will usually add more lane change sections.  Both the cars and the lane-change gates are powered by a single power supply.  Once you get above 4 or 5 lane change sections there will be a need for a track power upgrade providing more amperage to keep the electrical needs of the lane change gates from affecting the performance of the cars.  An alternative is to rewire the system to power the lane change gates separately from the cars.  This takes some skill and equipment for soldering.  Cost-wise this is probably not much different from adding a larger power supply or powering each lane separately on a multi-lane conventional layout, a very common practice.

The digital module in a Scalextric (L.) and in an SCX car.  The type of lane change activation, electric or mechanical, dictates the size, configuration, and constraints on the location of the module.

One manufacturer, SCX, powers its lane changers mechanically rather than electrically.  This takes the number of lane change tracks in your layout entirely out of the power equation.  This is great, but it’s bought at a price.  The mechanical lane changes require each car to have a bulky activator located immediately aft of the guide post, between the car’s front wheels.  This makes it difficult, and in far too many cases impossible to convert other manufacturers’ cars to the SCX system.  You can adapt almost any slot car to any digital system with enough skill, equipment, cost, and effort, but as a practical matter most users will be restricted to running only SCX cars on their SCX digital layout.